Demelza was born to Tom Carne, a miner, and his wife sometime in 1770. She had two brothers, Sam and Drake Carne. She came from a very poor family, and worked as a scullery maid. At some point, she adopted a dog she named Garrick. Her mother died sometime prior to 1783.
While at the market in 1783, she was attacked by a group of people trying to steal her dog and was saved by Ross Poldark. She speaks with Ross while he buys her a meal. Demelza explains to him about her abusive father so Ross says he will employ her as a scullery maid at his home in Nampara. Demelza agrees.
The next day, Demelza's father came looking for her. Her found her at Nampara, shortly before Ross came home. When he did, Ross told Prudie to send Demelza home. Ross then meets Tom, who tells Ross to stop trying to save her, thinking he seduced her. Tom has brought with him a group to help him fight Ross, so Jud goes to get help from some of their friends, and so, despite Demelza's protests, the two groups fight. Tom and his friends then flee in defeat.
Later on, Prudie tricks Demelza into hearing Ross complain about her, and how she is more trouble than she's worth. Demelza leaves, but Ross finds out and with a change of heart, goes to retrieve her. He finds her by the coastline and persuades her to return home with him.
Demelza finds having to clean herself all the time annoying, but Ross rebukes her complaining. She finds Jud and Prudie to be terrible to her, and they keep telling her they don't think she is welcome there.
One evening, Demelza sneaks into Ross's study and looks through many mining documents and maps and appears to share a love of them. She later goes out for a walk, and sees Ross's from afar in the ocean. She watches him, indicating that she has some affection for him.
The next day, Demelza accompanies Ross on a trip to town so she may buy some fish. While at home, Ross suddenly arrives back with Francis Poldark, who was wounded in a fight. Demelza helps patch Francis up, saving his life.
Demelza later agrees to help Ross, with Jud and Prudie, salvage his mining project, refusing to return home to her family when Ross gives her the opportunity, telling him she now belongs at Nampara.
A few months later, Demelza attended the re-opening of Wheal Leisure, passing round drinks and talking with many people. However, many of these people gossip about her relationship with Ross and believe that they are romantically involved even though they are not married.
When Prudie sprained her wrist, Demelza had to step in and prepare the meals too. She is very worried that she won't do well, but eventually manages to prepare a meal for Ross which he enjoys, to her delight.
When Jinny visits her to see if she can talk to Ross about Jim illegally hunting, even though he promised them he would stop, she goes to find him only to discover he is out. She takes the time to search the room and looks at papers on his desk. She then finds a trunk containing a teal dress. She then hears Ross calling for her, who wonders if she would like to have dinner with him that evening. She agrees. They discuss her work, and she tells him about Jim. Ross then tells her he will give Jim a better job than the one he has already, which makes Demelza happy.
However, Jinny visits them the next day to tell them Jim went on one final expedition and ended up getting arrested. Demelza stays at home to comfort Jinny, while Ross attends Jim's trial to persuade them to let Jim go.
While he is gone, Tom visits Demelza again. Tom has remarried, and wants to show Demelza he has changed. Tom says he wants Demelza to return home, and get away from Ross, who he thinks Demelza is sleeping with, but she refuses. Tom then states that he will return in one day to collect her.
Demelza is sad that she may have return home, and decides to try on the dress she found in the trunk to think about the life she could have had. She cries, stating that she cannot leave Ross, who then suddenly arrives home. He is also upset, because he couldn't help Jim, and when he notices Demelza is wearing the dress she found, he gets angry about her going through his belongings. They kiss, but Ross then stops, saying he did not employ her so he could sleep with her. He goes to his room, but Demelza follows him, claiming that she could not undo the dress. They then end up sleeping together.
The next day, Demelza takes a walk and returns home to find Ross working. Elizabeth Poldark, the ex-girlfriend of Ross, suddenly shows up and guesses something has happened between them. Demelza gets upset, and flees, believing that she should not stay and be a servant with Ross now that they have spent the night together, and proposes marriage to her. She agrees, and they have a small ceremony with Jud and Prudie in attendance.
Mistress of Nampara
The news of Demelza's marriage to Ross reaches many people in their village, but not everyone is happy about it because she was only a scullery maid. Jud and Prudie find it strange that Demelza suddenly shows up to be a new servant, only to become the lady of the house. Demelza is worried about Ross telling others, including his family, but he doesn't share her worries. Many are shocked, some are gleeful and snarky, believing the mining expedition will now fail, while Francis believes that they won't be welcome amongst polite society, which pleased Ross because he is fed up of the higher society. Only Verity Poldark is happy and writes to them, saying she would love to call by to tell them.
Demelza struggles with her newfound higher position as Mistress of Nampara. She is worried about Verity's visit because she does not know what to do. However, Ross brings her home many gifts and asks her to come see him at work one day. Demelza eventually agrees to, and finds that the workers like her. But she later says she doesn't want to go again, because she doesn't feel like the proper lady she is expected to be. To make her feel better, Ross says she may choose her own servants, so she employs Jinny to be the new kitchen maid.
Verity later pops over to Nampara for an extended visit. Ross is thrilled to see her, but Demelza’s anxious because she’s never really spent any time with Ross’s family. However, everything goes well. Verity is kind to her, and says she is glad Ross has married for love. Demelza then admits that she is in love with him too, but she doesn't think he will love her in quite the same way. She also tells Verity that she doesn't really know how to be a lady.
Verity helps educate Demelza about the ins and outs of being a nobleman’s wife, including the fine art of curtsying, how to set a table, successfully deploy a fan and do various types of dances. They later go shopping for new dresses and accessories, and Demelza tells her that she is pregnant.
Ross and Demelza recieve an invitation to spend Christmas at Trenwith. Demelza is worried about how she will be treated there, believing they will look down on her. Ross tells her she is being silly.
When they arrive, Verity greets them. Demelza looks awe-struck by the house, and is then greeted by Elizabeth who is surprisingly kind to her. However, Aunt Agatha spends the time trying to find out where Demelza is from, and makes her sit next to Elizabeth to see how she weighs up to her. But the meeting goes well until the Warleggans show up and make fun of them, even joking about Demelza being a maid. However, Demelza had gone up to her room to put on a new dress she brought for the meal.
At the meal, Ross is proud of Demelza and introduces her to everyone else. The meal is spent by the Warleggans making fun of them again, but Demelza later sings for the group. The next day, Ross tells Demelza he is in love with her, and she tells him she is pregnant.
Many months later, Demelza goes for a walk and then attends a show but decides to go home early which is when she goes into labour. She gives birth to a baby girl, and names her Julia Grace Poldark.
At Julia's christening, she is upset when Ross decides to invite everyone to Nampara for a gathering. She is worried about her friends and family, and his friends and family meeting, because they are very different. The Warleggans later show up to tease everyone, while her father and stepmother show up, only for Tom to make a scene.
Demelza later meets with Verity, and says she believes that she should seek out her old boyfriend, Captain Blamey, to find her own happiness. Verity is too scared to, so Demelza decides to seek him herself. She finds Blamey, but he won't admit he loves Verity and says he is too tied to his ship to be with her, and that he doesn't really think about her anymore. Demelza leaves.
However, Blamey later goes to Nampara and admits that he is still deeply in love with Verity and wants to be reunited with her. They make a plan to secretly meet her while she is out shopping. They go to town, but when Verity sees them, she flees. Demelza follows her and stops her but their talk is stopped by the miner's riot. They freeze, but Blamey saves them and they hide behind a nearby cart.
Demelza is talking to Ross, who is telling her about how people are gossiping about a supposed reunion between Verity and Blamey. Demelza remains coy, and doesn't say anything when Ross wonders how they two found each other again.
Verity later visits to tell Demelza that the Warleggans have invited everyone, including them, to a ball. Demelza is excited because she has never attended one before, and believes it's a great way to make her entrance into society. Verity helps her learn the latest dances, so she can make a good impression.
Later on, Demelza hears from Ross about Jim's death. She comforts him but is worried about the idea that he may be in trouble with the law for trying to help Jim. When Ross states they will not being attending the ball anymore, even though he ordered her a new dress, Demelza is left heartbroken. However, the next day, Verity makes Ross understand that they have to go, so that he makes them see he is still one of them, and they won't make a move against him. He agrees to go.
At the ball, Ross is drunk. Demelza spends most her time in the company of Verity until she leaves when Blamey shows up even though she had not told her family they are together. Demelza puts on a good face, and befriends Dwight Enys, who introduces her to a neighbour. Verity later cries to Demelza when Francis found out about her and Blamey and forbade them from meeting again.
When they return home, Demelza receives a note from Blamey requesting her help in making Verity make a choice about whether to be with him or stay with her family.
Demelza helps Blamey and Verity send letters to each other. She later finds out about Dwight Enys' relationship with Karen, and her murder. She is worried and believes Mark will be executed for killing her. Mark and a friend then show up at the house and say that he didn't mean to kill Karen and that she started the altercation by cheating on him. Mark says he needs to get away before he hurts someone else, like Dwight. They agree to help him, and arrange a boat to take Mark away from Nampara Cove. However, soldiers show up at their home looking for Mark, so Ross and Demelza entertain them while Mark hides, and they say they don't know where Mark may have gone.
Demelza recieves a note from Verity telling her that she is going to elope with Blamey, to Demelza's joy.
Demelza stays at home while Ross travels to Trenwith. Dwight pops by to find company, but Demelza knows he needs to go before Mark returns. It's too late, as Mark returns to hide because there are soldiers still searching for him. Demelza has to stop Mark and Dwight from fighting, stating she won't allow them to hurt each other in her home. Mark is worried that Dwight will tell the soliders where he is hiding, but Demelza points out that Dwight cannot say anything without getting her into trouble, and since Dwight is her friend, he won't do that. Eventually, Mark leaves Nampara for good.
Demelza then goes to Trenwith to admit that she helped Verity and Blamey elope. Francis is angry with her and tells her never to go to Trenwith again. Demelza later tells Ross what happened and he is upset with her, because her meddling made Francis so angry he told George Warleggan who they were partners with in their mining expedition, persuaded them to give their share to them, which means the Warleggans have demanded that they pay their loans. Demelza is sorry and begs him to forgive her, saying she only had Verity's best interests at heart, and Ross says he will try to forgive her. She says she won't be happy until Ross forgives her, but Ross says it will take some time.
Jinny later tells Demelza that there is a sickness in town, and that the servants of Trenwith have it. Demelza deduces that Francis, Elizabeth and their child may have it, and decides to go to Trenwith to help them. Francis forgives her and is grateful for her help. She saves them, but when she returns home to Nampara, she and Julia fall ill. Dwight believes that Demelza and Julia may not make it, and Julia dies that evening. Demelza keeps battling her illness, but she doesn't know about Julia since she is in and out of consciousness.
Demelza eventually pulls through, with Elizabeth and Dwight's help. She asks for her daughter, but Ross breaks down and gives her Julia's bracelet. Demelza realises she has died, and cries, asking whether Julia was afraid when she died.
Demelza sits on Julia's bed, and is sad she never got to say goodbye. Ross takes her for a walk to see the sunset, and they release Julia's bracelet into the wind. She tells Ross to make it up with Francis in light of everything, and rebuild Wheal Leisure. However, soldiers then show up to arrest Ross for wrecking, creating a riot and murder due to the word of George Warleggan. Demelza is helpless and watches on.