Flinn was born in Glen Cove, New York and grew up in Syosset, New York and Miami, Florida. At the age of five she started thinking about being a writer and submitted early efforts to magazines like Highlights, which did not publish them. At twelve, she moved to Palmetto Bay, Florida, a suburb of Miami, where she still lives. She had a hard time making friends at her new school, and she has said that this experience inspired much of her writing for young adults, particularly her book, Breaking Point. She graduated from Miami-Palmetto High School and was in a magnet performing arts program called PAVAC (Performing And Visual Arts Center), which inspired some of her book, Diva. She graduated from the University of Miami with a degree in vocal performance (opera) then went to law school and practiced law for 10 years before retiring to devote herself full-time to writing, following the acceptance of her third book. She is married to Eugene Flinn and has two daughters.

Breathing Underwater was Flinn's first novel. It was originally published in 2001 and was chosen a Top 10 ALA Best Books for Young Adults for that year. The story of 16-year-old Nicholas Andreas, a wealthy Miami teen who is sent to anger management because his girlfriend, Caitlin, takes out a restraining order against him. Ordered by the judge to write about his relationship, Nick tells of falling in love with her, and the eventual time when his anger took over and he hit her. The book is set in Miami and Key West, Florida. An excerpt from the book was included in Liz Claiborne, Inc's Love is Not Abuse curriculum, which was formulated to teach students about dating violence. Diva, a sequel to Breathing Underwater was released in 2006. It tells the story of Caitlin attending performing arts school as she gets over what happened between her and Nick and learns to be her own person while bonding with her often-flighty mother.

Nothing to Lose (2004) is about Michael Daye, a 17-year-old boy, who runs away from home after his mother killed his stepfather. Michael finds refuge at a traveling carnival and also falls in love with a fellow carny, Kirstie. But Michael finds out his mother is on trial for murder. He must decide whether to stay with the carnival and Kirstie, or come back to Miami to defend his mother. The book was chosen as a Booklist Top 10 Youth Mystery, and American Library Association Best Book for Young Adults and an American Library Association Quick Pick for Reluctant Young Adult Readers.

Fade to Black (2005) is perhaps Flinn's second-best known realistic novel and a frequent high school required read. Alex Crusan, a 16-year-old HIV positive high school junior, is attacked in his car early one morning and sent to the hospital. The novel is written in the perspective of three people: Alex Crusan, the victim; Clinton Cole, the suspect; and the witness, Daria Bickel, a young woman with Down's syndrome.

A Kiss in Time is Flinn's newest novel, released in 2009. A Junior Library Guild selection for 2009, the novel is a modern version of Sleeping Beauty, in which Talia, princess of Euphrasia, touches a spindle, falling into a deep sleep for three-hundred years. She wakes to the kiss of an American boy named Jack, on a tour of Europe. Since everyone in her kingdom is angry at her for touching the spindle, she persuades Jack to take her back to his Miami home, where she enters a modern world, replete with South Beach models and Jell-O shots. A humorous fish-out-of-water romance, Booklist magazine said in their review that "[f]ans of Meg Cabot’s Princess Diaries and Gail Carson Levine’s Ella Enchanted (1997) will embrace this charming, lightweight fantasy." Publisher's Weekly called the book, "clever and humorous," while HornBook magazine called it "satisfying."

Flinn has stated that her upcoming novel, titled Cloaked, is a melange of several fairy tales, including The Frog Prince, The Shoemaker and the Elves, and The Six Swans. Set in Miami, it tells the story of a teen who works at the shoe repair counter at a posh South Beach hotel until he is sent on a quest by a princess, whose brother has been turned into a frog and set loose in the Florida Keys. It will be released January 25, 2011.

Many of her books have made the American Library Association's Best Books for Young Adults lists, as well as Quick Picks for Reluctant Young Adult Readers. They have also received such teen-selected honors as the International Reading Association Young Adult Choices list (Breathing Underwater, Nothing to Lose, and Fade to Black). Flinn's books seem to appeal to teens who might otherwise prefer not to read, which is the charge of the Quick Picks for Reluctant Young Adult Readers list. Her books have also been nominated for numerous state awards. Breathing Underwater won the Maryland Black-Eyed Susan Award in 2004. Beastly is nominated for the 2009 Lone Star State (Texas) Award and recently won the teen-selected Detroit Public Library Author Day Award.