Janel Parrish first achieved fame in Hawaii as the winner of local talent contests. She got her start as a professional entertainer when she was cast as "Young Cosette" in the National Touring Company of Les Misérables, and later portrayed "Young Cosette" in the Broadway production as well; the role showcased both her acting and singing abilities. She subsequently appeared in several community theater productions in Hawaii—most notably as "Scout" in Manoa Valley Theatre's revival of To Kill a Mockingbird in 1998. Parrish got her first off-stage acting job as a minor role in 1999's four-part miniseries Too Rich: The Secret Life of Doris Duke. Shortly after, she appeared in two episodes of Baywatch as a young girl named Hina.
In 2000 she was featured in Disney's made-for-TV live-action incarnation of the Pinocchio story entitled Geppetto. She then went on to appear as the character Vanessa in the short-lived sitcom The O'Keefes, as well as appearing in many Disney shows and made-for-TV movies on the Disney channel. Along with appearances on The Bernie Mac Show, Zoey 101 and The O.C. Parrish performed in Bratz, as one of the main characters, Jade. It is her first role in a theatrical motion picture as well as her first starring role. She also landed a recurring role in the hit NBC series Heroes.
Parrish has a recurring role as Mona Vanderwaal in the hit ABC Family TV series Pretty Little Liars based on the book series by Sara Shepard. Mona is a formerly unpopular girl at Rosewood who longed to be accepted by Alison DiLaurentis's group despite being continuously mocked by her. After Alison's disappearance, Mona befriends Hanna Marin and they undergo an "extreme makeover" together. As a result, they both become popular students. However, Mona is secretly jealous of Hanna's friendship with the other Liars (Spencer, Aria and Emily) which leads her to do drastic things. In March 2012, Parrish was promoted to series regular for the third season. In August 2013, she won the "Choice TV Villain" award at the Teen Choice Awards.