Blonsky was born and raised in Great Neck, New York. She and her brother Joey are the children of Karen, a school aide, and Carl Blonsky, a municipal worker for the village water pollution control district. She is of Czech and Irish descent on her mother's side. She began singing at age three and started taking vocal lessons at the age of eight. She was raised Roman Catholic.
Blonsky attended Great Neck North Middle School, and attended John L. Miller Great Neck North High School for one year. Then she switched to Village School, an alternative high school with only 50 students. After school each day, she would attend the after-school theatre program at William A. Shine Great Neck South High School, where she participated in productions of Les Misérables, Sweeney Todd, Kiss Me, Kate, and the title role of the opera Carmen.
Blonsky said that she yearned to play the role of Tracy Turnblad since seeing Hairspray on Broadway on her 15th birthday. In 2006, while working at Cold Stone Creamery, she found out she had been cast in Adam Shankman's 2007 film adaptation.
Blonsky began work on her debut album in September 2007 and collaborated with alternative rock musician Duncan Sheik on a number of tracks.. Her first single, "On a High", a cover of one of Sheik's songs, was featured in Blonsky's Lifetime Original Movie Queen Sized. On December 18, 2007, she performed with the Turtle Creek Chorale in her first solo performance.
On June 22, 2008, she sang the American National Anthem in the pre-race ceremony at the 2008 Toyota/Save Mart 350 at Infineon Raceway.
In 2009, Blonsky made a guest appearance on Ugly Betty, in the episode "Dressed for Success." Blonsky also co-starred in the MTV mystery series Valemont in 2009. She later starred in the short lived ABC Family series Huge, which premiered in June 2010, but was cancelled three months later.
In 2013, Blonsky appeared in two episodes of the NBC Broadway drama series Smash. Later that year, she appeared in the film adaptation of Geography Club.
In July 2008, Blonsky and her parents were involved in a violent confrontation with the family of Bianca Golden, a former contestant on America's Next Top Model, at the Providenciales International Airport in the Turks and Caicos. Golden, Blonsky, and Blonsky's father were all charged with assault. That December, the charges against Blonsky and Golden were dropped.
In 2011, Blonsky earned a cosmetology license, and began working part-time as a hairstylist and make-up artist in her hometown of Great Neck, New York in between acting auditions.