Lemelin graduated in 2001 from the University of Pennsylvania, majoring in communications and minoring in English. During her junior year, she spent a year abroad at the University of New South Wales in Sydney, Australia, while also interning for Network 9 as a production assistant.

In 2007, she joined In-Motion Pictures as a Writer and Producer of independent films. She co-wrote screenplays for Absolute Fear (formerly known as Project Fear) and Special Ops (formerly titled Disarmed). In 2008, Lemelin began voice-over work, playing an animated character in the DreamWorks' Kung Fu Panda franchise, as well as voicing several commercials. She is most known for her series regular role on the Cartoon Network's popular cartoon, Young Justice. In November 2016, production began for the third season of the Young Justice series with Lemelin as a cast member.

In 2010, Lemelin joined the board of the nonprofit Angel City Pit Bulls. Since 2002, she has regularly volunteered with Los Angeles-based Free Arts for Abused Children, and she is a regular volunteer for Best Friends Animal Society, including for its Pup My Ride program, which transports small dogs from high-kill animal shelters to other parts of the United States where there is a greater demand for small dogs.

As of 2011, in addition to acting in several studio and independent films (Playdate, WER, Get the Gringo, Absolute Fear, and The Republic of Two), Lemelin has been cast in 12 consecutive TV pilots (several of which went to series), including Men At Work (TNT), $h*! My Dad Says (CBS), The Whole Truth (ABC), Bunker Hill (TNT), Canned (ABC), Good Behavior (ABC), Cavemen (ABC), The Funkhousers (ABC), as well as FOX's Worst Week of My Life, Dirtbags, and Titletown, and had a lead in the SciFi Channel's original TV movie/back door pilot Anonymous Rex based on the books by Eric Garcia. Lemelin has also guest-starred in many network shows, with comedic and dramatic roles on Bones (FOX), The Mentalist (CBS), Brothers and Sisters (ABC), The League (FX), The Closer (TNT), Melissa & Joey (ABC Family), CSI: Las Vegas (CBS), Malcolm in the Middle (Fox), Rules of Engagement (CBS), Run of the House (WB), Out of Practice (CBS) and The Mullets (UPN). Her network television debut was on the critically acclaimed but short-lived Fox show Undeclared, in which she was cast one month after moving to Los Angeles.