Following an injury to Michael Nelson that left Norwich short of defensive cover at centre-back, Paul Lambert signed Leon Barnett on loan from West Bromwich Albion on 26th August 2010 until 4th January 2010.

Barnett, 25, joined the Baggies from Luton Town in July 2007 for an initial fee of £2.5million and had since made 55 league and cup appearances for the Midlands side, netting three goals in the process. He had made a total of 78 appearances for the Hatters, and scored three goals. He had progressed from Luton's youth academy and had a loan spell as a youth with Aylesbury.

The 6ft 1in centre-back joined Coventry City on loan lin 2009/10 and made 20 appearances for the Sky Blues.

Lambert said "He's a quality player who has played Premier League football as well as having solid experience at Championship level, so I'm delighted we've been able to bring him in. "He's here until January so he will provide competition for places when Michael and Zak Whitbread are fit again, which I think you've got to have."

Barnett made his debut on 28th August 2010 in the 1-1 draw at Notts Forest

Impressing for Norwich, it was announced on 10th December 2010 that Norwich were to make Leon's stay at Carrow Road permanent in the January transfer-window having agreed a price with West Brom. However the following day, he conceded a penalty against Portsmouth and was then sent off, receiving a n additional two match ban for throwing the ball at the referee.

Leon signed permanently for Norwich in January 2011 but his season was cut short soon afterwards by injury. He ended the season with 26 appearances and 1 goal

Leon made 18 appearances, scoring once in 2011/12 as Norwich finished 12th in the Premier League

On 14th March 2013, Leon joined Malky Mackay's Cardiff on loan for a month, to get some first team football. He made his Cardiff debut on 17th March in a 2-0 win against Sheffield Wednesday. He helped them win the Championship title with Cardiff when they drew with Burnley on 20th April 2013. They secured promotion to the Premier League on 16th April 2013when they drew with Charlton. That result took Cardiff back to the top flight after a 51 year absence.

Barnett returned to Norwich at the end of April following injury to Michael Turner against Reading on 20th April. Barnett had played 8 times for Cardiff.

Barnett signed a three-year contract with recently-relegated Championship side Wigan Athletic on 1st August 2013. He made his debut on August 3rd against Barnsley away and scored in a 4-0 win as did fellow debutant Grant Holt

Barnett and Wigan were releaged to League One on 28 April 2015 when Rotherham beat Reading 2-1 consigning Wigan and Millwall to the two remaining relegation spots along with already relegated Blackpool. Barnett made 24 appearances in 2015/16 as Wigan secured a return to the Championship but was not in the matchday squad when they secured promotion with a 4-0 win at Blackpool on 30th April 2016.

Barnett was released by Wigan in July 2016 and signed a two year contract with Bury on 20th July 2016. He played 26 games before being released at the end of the 2016/17 season.

On 31st May 2017, Barnett signed for League One rival Northampton on a two year deal. He signed following the departures of Zander Diamond, Gabriel Zakuani and Rod McDonald. Barnett made 18 appearances, scoring once, in 2017/18 as Northampton were relegated to League Two.

On 26th November 2018, Barnett was forced to retire from playing due to a heart condition. The 32 year old defender revealed his heart rate reached almost 300 beats per minute during his final game and that it would be unsafe to contnue playing and training. He will stay at Northampton coaching in the academy and the first team.

Northampton were relegated from League One on 1st May 2021.