Mia Thermopolis is a fifteen-year-old tenth grade private school student who lives with her mother Helen Thermopolis and her cat, Fat Louie, in a renovated San Francisco firehouse. Her father, Philippe Renaldi (who had divorced her mother years ago but still kept in touch with Mia) died two months earlier in a car crash. Although Mia is an average student, she is very unpopular, but has two good friends: Lilly Moscovitz and Lilly's brother Michael Moscovitz. Her grandmother, Clarisse, contacts her, wishing to see her. During their visit, Clarisse reveals to Mia that she is the Princess of Genovia.
In it's sequel, Mia has graduated college and turned 21. At her 21st birthday party, she must dance with all the bachelors when a mysterious handsome man dances with her. Later, Mia is soon shocked to find out that she must marry someone is 30 days or she will not be able to rule Genovia as its queen. She is upset. Her grandmother meets with Viscount Mabrey and his nephew, Nicholas, who is the man Mia met the night of her birthday. Clarisse invites Nicholas to stay. She is mad and steps on his foot and leaves. Her grandmother helps her find different suitors. Lily, Mia, Charlotte, Clarisse, and Joe look through the different suitors and settle for one named Andrew Jacoby. Mia and Andrew meet and Mia thinks she likes him. They have a few more dates and them Andrew proposes to her and she accepts. Although she is engaged to Andrew, Nicholas is flirting with Mia. At first he does it because his uncle who is Viscount Mabrey tells him too, but soon realizes he likes her. The two kiss at The garden party and Mia's foot pops. She then runs away but Nicholas runs after and they fall into a fountain. Mia is later attempting to shoot arrows but doesn't do it right. Nicholas comes by whistling and Mia asks lily to distract Andrew so that she could talk to Nicholas. Nicholas helps her and her arrows goes threw the ring. Nicholas tells Mia he came to pack up his things, because he is leaving. Later that night, Nicholas is throwing pebbles at Mia's window and tells her to come down. She does and Lily covers for her. They run off together on horses and find a nice place to stay by a lake. They talk, dance, and then fall asleep. The next morning, there is a man in a boat in the lake and he was recording Mia and Nicholas. Mia thinks Nicholas set it up and runs away. The footage the man shot of her and Nicholas is already on TV when she gets back. She quickly goes to Andrew and says she is sorry. He grabs her and kisses her and admits she doesn't really feel anything when they kiss and she agrees. They then just decide that they made a commitment and are still going to get married. The next day is the wedding day and Mia is very confused and nervous. She is waiting to walk down the aisle and Joe tells her that Nicholas didn't send the man that recorded them. Viscount Mabrey goes to the wedding while Nicholas debates if he should go or not, but decides against. After Viscount Mabrey leaves, their maid tells Nicholas that Viscount Mabrey sent the man who recorded him and Mia, and Nicholas leaves to the wedding on his fathers bike. As Mia walks down the aisle, she stops and asks for a minute and runs out of the ceremony. Her grandmother runs after her and gives her the option to either go in there and get married or don't. And that it was her decision. She went back into the ceremony and said that she should be able to rule without a husband. Viscount Mabrey says that there must be and instructs that Mia marry his nephew. Nicholas enters and declines the offer. Nicholas leaves and Vicount Mabrey goes after him. Almost everyone is the ceremony agrees that Mia should be able to rule without a husband. The next day is her coronation, she is sitting in a chair when Nicholas comes in and tells Mia that he is in love with her and asks and she loves him back and she runs over to him and hugs him and then they kiss and her foot pops again. The next scene is her coronation and Mia is officially crowned Queen Mia of Genovia.
- In the original book series, Mia lives in Manhattan, not San Francisco.
- In the book series, Mia's surname is "Renaldo", not "Renaldi".
- In the book series, Mia's full name is Amelia Mignonette Grimaldi Thermopolis Renaldo.
- She is a vegetarian and donates money to Greenpeace.
- In the books, Mia's father is not only alive, but he is the one who tells her that she is a princess. She responds by running away to the penguin house in Central Park Zoo.
- Mia was actually the first princess to be crowned queen in a Disney film, the second being Elsa. However it is more widely known that Elsa is a queen due to her being a soon-to-be official member of the Disney Princess franchise.