The son of John Ritter and Nancy Morgan, he is known for his work in television series such as Joan of Arcadia (2003–2005), Gravity Falls (2012–2016), Another Period (2015–2018), Kevin (Probably) Saves the World (2017–2018), and Raising Dion (2019–2022), and Matlock (2024–).

His other credits include Swimfan (2002), Freddy vs. Jason (2003), Raise Your Voice (2004), Happy Endings (2005), The Education of Charlie Banks (2007), W. (2008), The Perfect Family (2011), Wild Canaries (2014), The Meddler (2015), Carrie Pilby (2016), Bitch (2017), and The Tale (2018).

Next, he played a young man struggling to come terms with his sexuality in the dark comedy Happy Endings (2005), with Stephen Hunter of The Washington Post commenting, "The acting in this ensemble is of such a high order that the movie simply takes you in and makes you feel these lives as real".

[6] Ritter's portrayal of an Ivy League college student in The Education of Charlie Banks (2007) was met with critical acclaim: The Hollywood Reporter referred to it as a "head-turner" performance that combined "a believably brooding intensity with a bad boy swagger that brings to mind [a] young Matt Dillon".

[12] His next project was the 2011 independent comedy A Bag of Hammers, where he starred as a felon who winds up taking care of a neglected child; in a mixed review, The Hollywood Reporter felt that its tone was "muddled", but believed it "starts off strong, offering funny, well timed dialogue between Sandvig [and] Ritter".

[13] Also that year, he co-starred in The Perfect Family, which critic Frank Scheck felt worked solely on the strength of its performances: "Turner is deeply sympathetic as the religious matriarch ... while Ritter and Deschanel are thoroughly believable as the grown children who love their mother despite her oft-expressed disapproval of their actions".

[14] Between 2012 and 2016, Ritter starred in the voice role of Dipper Pines on the Disney Channel animated series Gravity Falls, a show about a pair of twins deciphering the weird goings-on in the mysterious titular town.

In his review for RogerEbert.com, Brian Tallerico said, "What's kind of refreshing about About Alex is that the predictable map through the clichés of this subgenre that seems to be charted in the opening scenes isn't quite followed", adding, "Ritter finds [the] emotional undercurrent that's often missing from these troubled characters.

[22] Ritter had a supporting role as the ex-boyfriend of main character Lori in the 2015 comedy-drama The Meddler, and began playing Lord Frederick Bellacourt—an illiterate twin—that same year on the acclaimed historical sitcom Another Period,[23] a parody of Downton Abbey.

[26] Next, he starred as a man suffering from retrograde amnesia in the post-apocalyptic drama Embers (2015); Sight and Sound critic Anton Bitel called it "a reflection of the human condition as philosophically compelling as it is emotionally intelligent", adding, "No film has either moved or provoked me as much this year".

[28][29][30] In the satirical dark comedy Bitch (2017), Ritter re-teamed with director Marianna Palka to play Bill, a promiscuous husband whose wife suddenly takes on the mannerisms of a wild dog.

[37] In 2019, Ritter played the voice role of Ryder—a member of the Northuldra tribe—in Walt Disney's Frozen II, which broke box office records upon its release.

[38] He began starring as biotech engineer Pat Rollins on the critically acclaimed superhero series Raising Dion the same year,[39] with Aaron Pruner of Thrillist commenting that Ritter's involvement "adds the nuanced character work he's known for".

[41] In a casting choice that drew attention for being "meta",[42] Ritter appeared as Deputy Denny Reese in two episodes of the true crime Hulu miniseries Candy (2022), where the character—along with his colleague, played by Justin Timberlake—investigates the 1980 murder of Texan housewife Betty Gore.