Jonny Mullins was at the Norwich City Centre of Excellence at Potters Bar in the late 1990s until a change of rules by the Football Association forced Norwich to close the Centre due to travelling times.

Joining Reading's Academy, he spent two periods on loan at Kidderminster Harriers. The first from 17th December 2004 until 7th April 2005 in whcih he made 21 appearances scoring twice. The second from 25th October 2005 until the 2nd May 2006 in which he made 30 appearances, again scoring twice.

He was released by Reading in May 2006 and the Harriers expressed interest in signing him although they are facing competiion from Chester City. Instead he signed for Mansfield Town on a two year contract on 1st June 2006. "I wasn't going to be figuring much in the Reading side, so I see the move to Mansfield as nothing but positive," he told the Stags' website. "When the gaffer (Peter Shirtliff) contacted me, I was delighted to come back into league football."

Despite making 49 appearances during 2007/8, scoring three goals, and winning the Player of the Season award, Mullins could not help Mansfield avoid relegation out of the Football League and into the Conference

On 18th June 2008, John agre a two year contract with Stockport County