He has recorded for Truck Records in the UK, Sidecho Records in the USA, Big Mouth Records in Japan, and Shock Records in Australia. He was formerly known as MC Lars Horris but changed his pseudonym to MC Lars in 2004 for purposes of simplicity. MC Lars currently resides in California.
In 2006, he released the single "Download This Song" with it entering the Australian Singles Chart at number 29.
Nielsen appeared in the 2008 independent film Community College as MC Lars.
He attended Stevenson School and was the co-founder of the Monterey Bay Area punk rock band Amphoteric. He would later leave and the group would switch directions and gain a progressive metal following (especially amongst Internet listeners). Although the line-up has changed entirely, the group still resides in central California and continues to release albums independently. While at Stevenson he had a morning radio show through the school's radio station, KSPB. The show was called "Morning Madness", which featured Andrew and his co-host, Chris Gates.
Lars moved on as an English studies-major student of Stanford University in California who went on international study at Corpus Christi College, Oxford University in England. During his time at Stanford, MC Lars and fellow band member and Stanford student, Mike Love, co-created and appeared in Good Morning Tresidder Union which appeared on Stanford Cardinal Broadcasting Network (SCBN). While at Stanford, Lars drew a comic strip called 27th Street for the Stanford Daily. Following his graduation in 2005, it became a webcomic. He had a radio show on Stanford's radio station KZSU, playing nerdcore hip hop and old school rap, prior to being discovered by Truck Records.
As with many other rap and Hip-Hop acts, samples play a key role in MC Lars' music. But, with intent to preserve his rock roots, MC Lars chooses to sample alternative bands such as the British band Supergrass and Long Island, New York's Brand New.
MC Lars has also shown an interest in using lyrics and song titles based on English and American literature. "Rapbeth"'s title parodies that of William Shakespeare's play Macbeth whilst "Mr. Raven" is inspired by Edgar Allan Poe's "The Raven." Also, the song "Ahab", which features on his latest release The Graduate, sees Lars rapping about the novel Moby Dick.
The term iGeneration, used to describe the generation born primarily in the mid-to-late 1980s, was not actually coined by MC Lars, but it was used in his song of the same name, which was given out for free to Facebook users in August 2006 in conjunction with iTunes.
MC Lars' MySpace profile includes exclusive tracks related to the MySpace social scene of the iGeneration: "Internet Relationships", "Hot Topic is Not Punk Rock", "Download This Song" and "Signing Emo". Hearts That Hate, whose song "Cry Tonight" is sampled in Lars' "Signing Emo", is a fictional group created by the rapper. The Grammy-nominated Texan band Bowling for Soup performed as Hearts That Hate when MC Lars has supported them on tour. A full version of "Cry Tonight" is available as a B-side to the UK "Signing Emo" single.
In 2006, Lars worked with the Canadian independent record label Nettwerk Records to release The Graduate. Following this release, Lars toured extensively throughout the U.S. and the UK and even did shows in Japan, and Australia. Not long after the album was released on iTunes, Lars received an e-mail from a 15-year-old fan Elisa Greubel on his web forum saying she identified with "Download This Song" since her family was one of many being sued by the Recording Industry Association of America. This led to Nettwerk executive Terry McBride and a team of major-label artists managed by Nettwerk to support the Greubels with their case.
MC Lars returned to the UK in October 2006, due to popular demand, to play more shows in support of his "Ahab" single and video.
In April 2007, MC Lars and Nettwerk made the component tracks for his single "White Kids Aren't Hyphy" available for remix under a by-nc-sa Creative Commons license on the Jamglue online mixing site as a contest. In May, MC Lars toured the UK on the third installment of the Good To Go Tour, making friends with Wheatus front man Brendan B. Brown. In July Lars and Brendan recorded a bunch of new songs, two of which would later appear on This Gigantic Robot Kills. In November, Lars returned to the UK on tour with pop-punk band Last Letter Read who performed their own set and then on stage with Lars, debuting Lars' new song "Hey There Ophelia".
In 2008, Lars worked with "Weird Al" Yankovic, the Rondo Brothers, Nick Rowe and Mike Kennedy of Bloodsimple, Daniel Dart of Time Again, Donal Finn of Flash Bastard, Pierre Bouviere of Simple Plan, MC Bat Commander of The Aquabats, Suburban Legends, Worm Quartet, Gabriel Saporta of Cobra Starship, Brett Anderson of The Donnas, MC Frontalot, Jesse Dangerously, Jaret Reddick of Bowling for Soup, Linus Dotson of Size 14, Parry Gripp of Nerf Herder, Jonathan Coulton, Aesias Finale, Sebastian Reynolds, Joe Ragosta of Patent Pending and classical musician Walt Ribiero to complete his album "This Gigantic Robot Kills".