Born in Northwood, north-west London, Cotton grew up in the nearby suburb of Eastcote, in Abbotsbury Gardens. She was brought up by Lyn and Mick Cotton, and has a younger brother called Jamie. Although her grandfather's cousin is former BBC executive Sir Bill Cotton, she says she has not seen him since the age of five and cites her rise to broadcasting fame as down to her own hard work and discipline. She attended Newnham Primary School in Eastcote/Eastcote Village. She read Art at university.

In 2003 Cotton started presenting Top of the Pops:Saturday (later called Top of the Pops: Reloaded) with her The Saturday Show co-presenter, Simon Grant, which then later became Sam and Mark from Pop Idol and JK and Joel. From 2004 to 2006 Cotton was the main presenter of Top Of The Pops, which she co-presented with a different guest presenter each week. She was one of the final ever regular presenters for that programme. The programme ended in July 2006, officially as a result of poor viewing figures, but Cotton was unavailable for the final episode because of her filming for ITV's Love Island in Fiji. However, a quick introduction she recorded on location was shown at the start of the broadcast.

In September 2005 she joined BBC Radio 1, to present the Friday early morning slot with Reggie Yates[2] who she had previously worked with on CBBC as a presenter, and co-star of Only in America and Top Of The Pops. She occasionally stands in for other Radio 1 presenters, such as Sara Cox. It was announced on 2006-07-30 that Cotton would be co-hosting the weekend breakfast broadcast along with Reggie from September 2006 replacing Spoony, who was leaving the station after six years. From October 14, 2007, Fearne and Reggie presents the Radio 1 Chart Show, taking over from JK & Joel, who left the station. Cotton became the first ever permanent female presenter of a BBC radio chart show.

On 17 March 2007 she, along with Terry Wogan, hosted Making Your Mind Up 2007, the show to decide which song the UK will send to the Eurovision Song Contest 2007 in Helsinki, Finland. On the night Cotton saved the blushes of her co-presenter when Wogan accidentally announced the wrong artist as winner. The Eurovision Song Contest, held in Helsinki, Finland on 12 May 2007, saw Cotton return to Europe's screens as the face for the UK vote. It fell to her to announce the highest award of 12 points to Turkey.

In January 2008, it was confirmed that Fearne would be heading to the US to host The Guinness Book of Records - Live, proving her worth over a number of American candidates. Following this successful first US appearance, Fearne secured a job co-hosting the NBC reality show Last Comic Standing. It was announced that due to this she had quit her presenting role on The X Factor spin-off The Xtra Factor.