He signed for Newcastle on a permanent basis in January 2010, before relocating to Queens Park Rangers on a free transfer in June 2013.

In early January 2006, he was sent on loan, along with three other United youths, to the Belgian Second Division club Royal Antwerp for the remainder of the season, to gain match experience.

[14][15] Simpson made his Blackburn debut on 27 August 2008, playing in Rovers' 4–1 win over Grimsby Town in the League Cup Second Round.

[19] On 14 August 2009, Simpson joined Newcastle United on loan until January 2010,[20] making his debut in a 1–0 victory over Sheffield Wednesday at St. James' Park.

An undisclosed fee, reported as being in the region of £750,000,[21] was agreed between Newcastle and Manchester United and Simpson signed a three-and-a-half-year contract on 20 January 2010.

[22] He played a solid part in Newcastle's promotion to the Premier League, winning his second Championship title in the process, while forming a right flank partnership with January signing Wayne Routledge.

On 20 September 2011, Simpson scored a curler in Newcastle's 4–3 win over Championship side Nottingham Forest in the third round of the League Cup.

On 10 December 2011, Simpson started alongside James Perch at center back in place of the injured Steven Taylor and Fabricio Coloccini for Newcastle's 4–2 defeat at Norwich City.

After his contract with Newcastle expired at the end of the 2012–13 season, Simpson signed a three-year deal with Championship club Queens Park Rangers on 27 June 2013.

[citation needed] On 30 August 2014 signed a three-year contract for fellow promoted team Leicester City for an undisclosed fee.

However, he didn't make his second appearance for the club until 7 December against Aston Villa, as he often struggled to get into the side ahead of regular right back Ritchie De Laet.

[30] In February and March 2021, Simpson began playing with the Leicester City U23s squad in order to regain match fitness.

Despite this, he confirmed in a December 2022 interview, whilst launching a campaign to better support players' mental health at the end of their careers, that he was not yet retired, and was still training in the hope of finding a new club.

[36] By October 2023, he had returned to Manchester United to study for his coaching badges whilst weighing up the next step in his career, alongside fellow ex-United player Phil Jones.

[39] On 7 August 2024, it was announced that Simpson would partake in an exhibition boxing match headlining MF & DAZN: X Series 17 at the 3Arena in Dublin, Ireland against English YouTuber and streamer Danny Aarons.