Lengies was born in 1985 in Montréal, Québec. Lengies grew up in the small town of Hudson, Québec. There she graduated from Hudson High School in 2002.
Lengies got her start in Canadian television on shows such as Sponk!, Are You Afraid of the Dark?, Radio Active, and Popular Mechanics for Kids. She lent her voice to the character Emily on the PBS animated series Arthur.
In 2000, she had the lead role in the Showtime film Ratz.
In 2002, Lengies was cast as a series regular in the NBC comedy-drama American Dreams, set in Philadelphia in the mid-1960s, playing teenager Roxanne Bojarski, who becomes one of the dancers on the American Bandstand television show hosted by Dick Clark. The series ran for three seasons, with the final episode broadcast in March 2005.
In August 2005, Lengies co-starred with Hilary Duff and Heather Locklear in the comedy The Perfect Man. She also played a supporting role as the jailbait hostess, Natasha, in the 2005 American film Waiting..., and reprised the role in the film's 2009 sequel, Still Waiting....
In 2006, she co-starred with Jeff Bridges and Missy Peregrym as a gymnast in the film Stick It. For The Grudge 2, the role of Vanessa was originally written for Lengies, who eventually turned it down to film My Suicide; the part still bears her name. She has also appeared in CBS show Ghost Whisperer in an episode titled "The Vanishing" and the NBC show Medium in the episode "Apocalypse... Now?". She made another appearance in an episode of the short-lived CBS show Moonlight.
Lengies appeared as Sophia in the Lifetime drama series Monarch Cove. She also co-starred in the ABC online comedy, Squeegees.
Starting in 2009, Lengies appeared as Nurse Kelly Epson on the TNT medical drama Hawthorne. The role was a recurring one for the first season, and Lengies became a series regular for the following two seasons. The show, which ran for three seasons of ten episodes starting each June, was not renewed for a fourth summer.
In August 2011, Lengies was cast in the recurring role of Sugar Motta for the third season of Glee. Sugar, who is well-off, self-confident, and has a tin ear, first appeared in the season premiere on September 20, 2011. Since then, Sugar's singing greatly improved, and she started to perform with the main glee club, New Directions.
In 2012, she appeared in the recurring role of Loco Uno in an online mockumentary and sitcom called MyMusic, produced by The Fine Bros.
In 2013, Lengies visited her old high school, Hudson High School (now Westwood Senior High School) to see how much it had changed, and to speak to the Student Life and Drama students about her journey of how she got to be cast on Glee. Following the end of the fourth season, Lengies announced her departure from Glee, citing frustration with not having the opportunity to act as she was hired per episode and was cast as Kacey, a bubbly cocktail waitress, in the ABC sitcom Mixology. However, ABC canceled Mixology after only one season.
In 2015, Lengies returned to Glee for two episodes of the final season. Later that year, Lengies was cast in a guest role on FOX's 2016 drama Second Chance (earlier titled Lookinglass), and was promoted to the main cast after the first episode aired.
In 2016, Elijah Adams offered the part of Lady Hatt in the upcoming EATL Films series The Railway Series to Lengies, who didn't accept the role due to her commitment with Second Chance. On May 12, 2016, Second Chance was cancelled by FOX. From June 20, 2016 to August 29, 2016, Lengies appeared as Kordi Freemaker, a scavenger in the first season of the Disney XD series LEGO Star Wars: The Freemaker Adventures. She will be reprising her role in the second season, premiering in the summer of 2017.
In 2017, Lengies was re-approached by Adams about portraying the character. The two were in negotiations for nearly three months, but Lengies once again declined to accept the role.