Her breakthrough role was Jane Margolis on the AMC drama series Breaking Bad (2009–2010), a character she reprised in its spinoff film El Camino (2019).

[9] (Ritter later starred in the 2011 premiere of Zach Braff's play All New People, also at Second Stage, co-starring Anna Camp, David Wilson Barnes and Justin Bartha and directed by Peter DuBois.

)[10] Ritter's early guest starring and recurring roles on television included Gia Goodman, the daughter of Mayor Woody Goodman (Steve Guttenberg) on the second season of Veronica Mars; Rory Gilmore's friend, Lucy, on Gilmore Girls for eight episodes in 2006–07; and the first iteration of Allison Stark on the Fox sitcom 'Til Death (a role eventually played by four different actresses through the show's run).

The episode was a backdoor pilot for a proposed spin-off series, set in 1980s Los Angeles; intended to chronicle the teenage years of character Lily van der Woodsen.

[11] The role of Jane was her breakthrough, at which Ritter expressed surprise on People's Couch Surfing program: "Talk about cult following, this show really is the gift that keeps on giving.

[17] Ritter starred in 2010 as the sharp and quirky Lily in the Starz television series Gravity, alongside Ivan Sergei, Ving Rhames and Rachel Hunter.

That same year, Ritter appeared alongside Alicia Silverstone and Sigourney Weaver in the comedy horror film Vamps, written and directed by Amy Heckerling.

She plays a Manhattan socialite turned into a vampire.In February 2011, Ritter landed the lead role in the ABC situation comedy Don't Trust the B---- in Apartment 23.

She starred as Chloe, a New York City party girl and con artist who attempts to rip off her new roommates after they move in, but befriends and mentors one of the applicants.

[25][26] Her casting as aerospace engineer Dr. Mary Kendricks in the astronaut-themed comedy Mission Control was announced by the network in February 2014;[27][28] but on October 15, NBC said it was not moving forward with the show.

[29] A July 9, 2013, press release stated Ritter would star in Jake Hoffman's directorial debut, Asthma, about the indie rock scene in New York City.

[30] On December 5, 2014, Ritter was cast to star in the Marvel Television series Jessica Jones in the title role, as a former superhero turned private investigator.