Before his hosting career, O'Brien was a writer for the NBC sketch comedy series Saturday Night Live from 1988 to 1991, and the Fox animated sitcom The Simpsons from 1991 to 1993.

O'Brien was a writer and producer for The Simpsons for two seasons until he was selected by Lorne Michaels and NBC to take over David Letterman's position as host of Late Night in 1993.

[4] Known for his spontaneous hosting style, which has been characterized by The New York Times as "awkward, self-deprecating humor", O'Brien's late-night programs combine the "lewd and wacky with more elegant, narrative-driven short films".

[8] He was a congressional intern for Congressmen Robert Drinan and Barney Frank, and in his senior year won the National Council of Teachers of English writing contest with his short story "To Bury the Living".

[24] After graduating from Harvard, O'Brien moved to Los Angeles to join the writing staff of HBO's sketch comedy series Not Necessarily the News, where he worked for two seasons.

[29] During his three years on SNL, he wrote such recurring sketches as "Mr. Short-Term memory" and "The Girl Watchers"; the latter was first performed by Tom Hanks and Jon Lovitz.

Literally, in my first ten minutes at The Simpsons, a bird had flown through the glass of my window, hit the far wall, broken its neck, and fallen dead on the floor.

In his speech given at Class Day at Harvard in 2000,[45] O'Brien credited The Simpsons with saving him, a reference to the career slump he was experiencing before being hired for the show.

[41]: 162 [47] He fit in quickly, commanding control of the room frequently; writer Josh Weinstein called it a "ten-hour Conan show, nonstop".

[49][50] Meanwhile, David Letterman was preparing to leave the talk show Late Night, prompting executive producer Lorne Michaels to search for a new host.

[41]: 165  Wolodarksky recalled the experience: "Seeing this friend of yours, this guy that you worked with, walk out from behind that curtain and deliver a monologue was like something you could only dream up that you couldn't ever imagine actually happening.

[56] After O'Brien's departure, the writers at The Simpsons would watch videotaped episodes of Late Night at lunch the day following their midnight broadcast and analyze them.

[41]: 166–167 Late Night with Conan O'Brien, originating from Studio 6A at 30 Rockefeller Plaza in New York City, premiered on September 13, 1993, to unfavorable reviews from contemporary critics.

[61] Critics attacked O'Brien: Tom Shales of The Washington Post suggested that "the host resume his previous identity, Conan O'Blivion.

"[5][62] Generally, critics viewed O'Brien as nervous and fidgety on-camera, and that he was "too smart, too East Coast, too sophisticated, too young and even too tall to be successful.

The lips and voice of these characters (Clutch Cargo) – frequently a party-crazed "hillbilly" interpretation of Bill Clinton – were supplied by writing partner Robert Smigel.

"[69] O'Brien's audience, largely young and male (a coveted demographic), grew steadily and the show began to best competitors in the ratings, and continued to do so for 15 seasons.

[42] As of October 2005[update], Late Night with Conan O'Brien had for eleven years consistently attracted an audience averaging about 2.5 million viewers.

"[5][73] As part of the five-day trip, which was released as a one-hour special episode of Late Night, O'Brien met with Halonen at the Finnish Presidential Palace.

[83] Hosting The Tonight Show was a lifelong dream of O'Brien's, and the promise of succeeding Leno kept him in NBC's employ despite the fact that he likely could have secured a more lucrative deal at another network.

"[88] During the taping of the Friday, September 25, 2009, episode of The Tonight Show, O'Brien suffered a mild concussion after he slipped and hit his head while running a race as part of a comedy sketch with guest Teri Hatcher.

[96] On January 12, O'Brien released this statement: "I sincerely believe that delaying The Tonight Show into the next day to accommodate another comedy program will seriously damage what I consider to be the greatest franchise in the history of broadcasting.

Ferrell's wife, Viveca Paulin, together with Ben Harper, Beck, and ZZ Top guitarist Billy Gibbons, also joined the band for this final performance.

[108] Some industry insiders have speculated that O'Brien had chosen to stay on the west coast in order to facilitate a return to late night television and because he did not want to put his children through another move.

[144] The final show aired on June 24, 2021, featuring a live audience and marking the end of O'Brien's twenty-eight year run as a late-night host.

[152] O'Brien made a cameo appearance as himself on the February 26, 2022 episode of Saturday Night Live, as a guest inducting John Mulaney into the Five-Timers Club with Tina Fey, Steve Martin, Paul Rudd, Candice Bergen and Elliott Gould.

[158][159] On April 18, 2024, HBO released a four-episode international travel series titled Conan O'Brien Must Go on Max to widespread critical acclaim.

[184] O'Brien also made a guest appearance as the "Wandering MC" in the 2019 video game Death Stranding,[185][186] where he communicates with the player using voice lines and facial expressions recorded during his visit to Kojima Productions' headquarters.

[237][238] In January 2008, after his show was put on hold for two months owing to the strike by the Writers Guild of America, he reemerged on late-night TV sporting a beard, which guest Tom Brokaw described as making him look like "a draft dodger from the Civil War.

[240] O'Brien purchased a $10.5-million mansion in Brentwood, Los Angeles, California, to prepare for his move there in 2009 from New York City to host The Tonight Show at Universal Studios Hollywood.