She took up tap, ballet, and jazz dance from a young age and continued singing in the choir while in high school as well as performing in local theater productions. She went on to New York University and then moved out to Los Angeles.
She has made many guest appearances on TV shows such as One Tree Hill, CSI: Miami, and How I Met Your Mother, among others. She also had regular parts in My Alibi and Glory Daze.
In 2009, Guill appeared in the independent comedy, Costa Rican Summer. She also took on more minor roles in the films 2 Dudes and a Dream and Fired Up. She landed a leading role in the film Road Trip: Beer Pong, which went straight to DVD. Still in the same year, she was in a modern take on Friday the 13th and My Super Psycho Sweet 16, which was a made-for-TV film which premiered on MTV. The film earned good ratings for the premiere, but was met with mixed reviews from critics.
She stayed busy in 2009, taking on parts in The Apparition and Altitude, which was released in 2010 and in which she had a lead role. 2011 saw her co-star in Crazy, Stupid, Love alongside Steve Carell. In the same year, she made guest appearances on the TV shows Community and Psych.
Guill is friends with Community star, Alison Brie who she co-starred with in My Alibi. The two of them along with fellow My Alibi co-star, Cyrina Fiallo, started a three-person band called The Girls.