Graynor first came to prominence as Caitlin Rucker on HBO's The Sopranos. Her film credits include An American Crime (2007), which premiered in January 2007 at the Sundance Film Festival. She also appeared early in the second season of UPN's Veronica Mars as the daughter of a bus driver. She made her Broadway debut as 'Alison' in 2005's Brooklyn Boy after appearing in the world premiere at South Coast Repertory. She also appeared in The Little Dog Laughed.

Graynor played Elvina, a pop star, in CSI: Miami, and was a recurring guest star on the Fox series Fringe, playing Agent Olivia Dunham's younger sister, Rachel. In 2008, Graynor appeared in the film Nick and Norah's Infinite Playlist, and in October 2009, she also was in Whip It, a comedy film directed by Drew Barrymore and written by Shauna Cross, based on Cross' young adult novel Derby Girl.

In 2010 she appeared in the play Trust at the Off-Broadway Second Stage theatre company with Sutton Foster, Zach Braff and Bobby Cannavale. In the fall of 2011, she appeared on Broadway in the Woody Allen-written segment of three one-act comedies collectively called Relatively Speaking. She played Nina Roth in Allen’s “Honeymoon Motel” segment.

In 2012, Graynor starred in the comedy For a Good Time, Call..., and is also credited as the film's executive Producer. Graynor starred on Broadway opposite Cheyenne Jackson, Henry Winkler, and Alicia Silverstone in David West Read's play The Performers, which opened November 2012 at the Longacre Theatre.

During a June 29, 2017, CBS late night interview with Stephen Colbert, promoting I'm Dying Up Here, she mentioned she went to a prom with Future congressman Joseph Kennedy III.