Day first became interested in music in 2005 when he bought an electric guitar at the age of 15. He started his YouTube account one year later on 4 August 2006. In 2008, Day founded Chameleon Circuit, a Trock (short for "timelord rock") band that wrote music inspired by the British TV show Doctor Who. In June 2009, the band released its first self-titled album by YouTube musicians' record label DFTBA Records and a second album Still Got Legs in July 2011. Wired noted that Chameleon Circuit "puts out titles that could pass for chart-toppers – if they weren't excessively nerdy in the subject matter department."

Day also appeared with the Chartjackers at the Hammersmith Apollo for the BBC Switch Live 2009 show, along with artists such as Black Eyed Peas, N-Dubz, and Pixie Lott. Day was also a member of Youstage, an ad-hoc group of YouTube vloggers who competed successfully on the BBC Three talent show Upstaged during March 2008.

Day has received millions of plays for his music by releasing music videos on YouTube, with his most popular being the 2011 single "Forever Yours", whose music video reached 1.5 million views in its first week of release. The video has since been removed from the website by Charlie McDonnell.

On 16 December 2012, he attempted to break a world record with his song "Stupid Stupid". The song later peaked on UK charts at No. 25. In order to promote the single, Day held a one-day tour entitled Stupidfest, which was free for entry.

Day was called "one of Britain's most popular YouTubers." He created his first YouTube channel, nerimon, as a teenager making videos in his bedroom, stating, "I was making a video podcast to entertain family and friends – just a little comedy series."

In late 2013, Day announced that he had completed work on his first book; with the working title The Underground Storyteller, the book was about his experiences with the London Underground. It was twelve chapters in length and was scheduled to be published in July 2014. It was cancelled by the publishers. On 13 October 2014, Day announced online that he was releasing his book independently, keeping the name of The Underground Storyteller, and that he will sign and number the first thousand copies.

On 4 September 2015, Day announced he planned to publish a fiction book.

In December 2015, Day announced he had recorded five new songs for an untitled studio release in the Future. On 28 January 2016 he confirmed the album would be called Split Infinities and would be the first of his solo albums to feature a backing band.

Day unlisted a video about cosplaying entitled "Big Girls in Costumes" after criticism that he was belittling obese women who took part in the activity. He stated that it was "satirical" and that, "I’m not going to take the video down because I don’t want to pretend this didn’t happen - running away from mistakes isn’t how you solve them - but I have made the video unlisted so you can only see it if you have the link. I think that’s a good compromise between not risking more people being hurt by the content but also not trying to hide the mistake. I’ve also taken the ads off it."