She is best known for co-writing, co-producing, and starring in the 2002 indie film Kissing Jessica Stein,[3] for which she received an Independent Spirit Award nomination for Best First Screenplay.

[4] Westfeldt's television work includes recurring roles on Notes from the Underbelly (2007–2009), Grey's Anatomy (2009), 24 (2010), and Younger (2017–2021) as well as guest starred on shows such as Girls, and This Is Us.

Westfeldt made her Broadway debut in 2003 in the revival of Wonderful Town[5] earning a nomination for the Tony Award for Best Featured Actress in a Musical.

[10] In 1997, she was cast as a series regular on the 20th Century Fox/ABC sitcom Two Guys, a Girl and a Pizza Place, alongside Ryan Reynolds, Traylor Howard and Richard Ruccolo.

[11] In 1997 Westfeldt co-wrote and co-starred with Heather Juergensen in an Off-Broadway play based on a series of sketches called Lipschtick: The Story of Two Women Seeking The Perfect Shade, which caught the attention of major Hollywood studios.

[14] Variety wrote, "A fresh take on sex and the single girl, this buoyant, well-crafted romantic comedy blends pitch-perfect performances with deliciously smart writing.

In 2020, Kvellar writer Mara Reinstein looked back on the impact that this "authentic" and "nuanced" movie had, writing, "I can't overstate how much Kissing Jessica Stein pioneered the gay rom-com.

"[21] Ben Brantley of the New York Times wrote, "Jennifer Westfeldt makes a charming Broadway debut as Eileen, Ruth's boy-magnet of a sister."

Andrew Johnston wrote in TimeOut: "Jennifer Westfeldt is a total babe, with physical-comedy skills that, if properly honed, could approach Lucille Ball territory.

[Westfeldt] gets to show her serious side, too, and if the series gives her enough exposure to start competing with Hope Davis and Laura Linney for blond-WASP roles, Underbelly will have more than justified its existence.

[citation needed] In 2011, Westfeldt wrote, produced, starred in, and made her directorial debut in Friends with Kids, which was a breakout hit at the 2011 Toronto International Film Festival.

Westfeldt starred opposite Adam Scott, with a cast including Maya Rudolph, Kristen Wiig, Chris O'Dowd, Ed Burns, Megan Fox and Jon Hamm.

Peter Travers of Rolling Stone called Friends with Kids "an indelibly funny and touching comedy with a real sting in its tail," and deemed Westfeldt "an actress of rare wit and grace, and now a filmmaker with a keen eye for nuance.

[33] Her varied TV credits include memorable arcs on Younger, Queen America (opposite Catherine Zeta-Jones), Grey's Anatomy, 24, Judging Amy, and Hack; and guest-starring turns on NBC's hit drama This Is Us, HBO's Girls, and Childrens Hospital, among others.