Louis Sebastian Theroux was born in Singapore on 20 May 1970, the son of American travel writer and novelist Paul Theroux and his English then-wife Anne (née Castle). His paternal grandmother, Anne (née Dittami), was an Italian-American grammar school teacher, and his paternal grandfather, Albert Eugène Theroux, was French-Canadian and a salesman for the American Leather Oak company. He holds both British and American citizenship. His older brother, Marcel, is a writer and television presenter. His cousin, Justin, is an actor and screenwriter. Theroux is the nephew of novelist Alexander Theroux and writer Peter Theroux.

Theroux’s first employment as a journalist was in the United States with Metro Silicon Valley, an alternative free weekly newspaper in San Jose, California. In 1992, he was hired as a writer for Spy magazine. He also worked as a correspondent on Michael Moore’s TV Nation series, for which he provided segments on off-beat cultural subjects, including selling Avon to women in the Amazon Rainforest, the Jerusalem syndrome, and attempts by the Ku Klux Klan to rebrand itself as a civil rights group for white people.

As part of the Weird Weekends episode “Porn”, Theroux agreed to film a cameo in the 1997 gay pornography film Take a Peak. He did not perform sexual acts in the film, but made a brief appearance as a park ranger in search of a criminal. In the Weird Weekends episode “Infomercials”, he was featured as a live salesman for an at-home paper shredder for the Home Shopping Network.

In these special programmes, beginning in 2003, Theroux returned to American themes, working at feature-length and in a more natural way. In March 2006, he signed a new deal with the BBC to make 10 films over the course of three years. Subjects for the specials include criminal gangs in Lagos, Neo-Nazis in America, ultra-Zionists in Israel. He also visits child psychiatry, and the prison systems in California and Florida. A 2007 special, The Most Hated Family in America, received strong critical praise from the international media.

In the series When Louis Met… (2000–02), Theroux accompanied a different British celebrity in each programme in their daily lives, interviewing them as they go. His episode about British entertainer Jimmy Savile, When Louis Met Jimmy, was voted one of the top documentaries of all time in a 2005 survey by Britain’s Channel 4. Some years after the episode was filmed, the NSPCC described Savile as one of the most prolific sex offenders in Great Britain.

Theroux published his first book, The Call of the Weird: Travels in American Subcultures, in Britain in 2005. In it he recounts his return to the United States to learn about the lives of some of the people he had featured in his television programmes.

While filming a 2011 BBC programme, Theroux was asked “Why pose a difference between religion and ethics?” He responded, “Because I don’t believe in God.” In his 2011 documentary, The Ultra Zionists, he confirmed his atheism. In a 2012 masterclass, Theroux spoke of the challenges of combining family life with the need to go away to work on projects.

Theroux married longtime girlfriend Nancy Strang on 13 July 2012. They have three sons. He and his family lived in Harlesden, London until they temporarily moved to Los Angeles, California in early 2013, allowing him more time to focus on his LA Stories series. In August 2017, Theroux again relocated to Los Angeles.

In an interview in 2015, Theroux expressed his intention to produce a follow-up documentary about Savile for the BBC to explore how the late entertainer had continued his abuse for so long, to meet people he knew closely, and examine his own reflections on his inability to dig more deeply into the first case. This follow-up documentary, with the title Savile, aired on BBC Two on Sunday, 2 October 2016, and lasted 1 hour, 15 minutes.

In December 2015 Theroux captained the team representing Magdalen College, Oxford, on BBC Four’s Christmas University Challenge. In their first-round match the team beat University of Exeter by 220 to 130 and Theroux’s team went on to win the tournament.

In October 2016, Theroux premiered a feature length documentary entitled My Scientology Movie. Produced by Simon Chinn—a schoolfriend of Theroux’s—and directed by John Dower, the film covers Theroux attempting to gain access to the secretive Church of Scientology. The film premiered at the London Film Festival in 2015 and was released in cinemas in the UK on 7 October 2016.

During a 2018 interview with The Guardian, he revealed that he was a nervous flyer.

Theroux released an autobiography titled Gotta Get Theroux This in September 2019.