At the age of 9, Ashton joined the Leicester City youth ranks, where he remained until signing as a trainee and then as a full-time professional. He played a part in the Leicester team who won promotion into the Premiership in the 2002/03 season. In November 2002, Jon went out on loan to Notts County, where he made 4 appearances. Then 19-year-old signed a new three-year contract with The Foxes in May 2002, however Jon did not see out the end of this contract at newly built Walkers Stadium.
Ashton was loaned out to Oxford United at the start of the 2003 season, he was so impressive in his first seven games for The Yellows, the club moved quickly to make the deal permanent at the end of his initial month. The young defender made over 100 appearances during 3 season at the Kassam Stadium.
Following both Oxford and Rushden relegation into the Nationwide Conference, Ashton signed for The Diamonds under Paul Hart in the summer, 2006. He started off his Rushden career as a central holding midfielder, moving back to central defender in order to cover for persistent injuries. Jon made 40 starts for Rushden in little over a year, including both 3-0 defeats, at Nene Park and The New Rec against new club, Grays Athletic.
Rushden & Diamonds boss and former Grays Athletic Director of Football, Garry Hill, who lasted just hours at the club, transfer listed Ashton in the close season. Jon signed a two-year deal with Grays Ath on the 30th May 2007.
Jon was selected in the England National XI squad in the close season of 2006/07 to face Finland.
In January 2009, the defender was transfer listed after informing the club he would not be staying at the club beyond his current contract. A few days before the transfer window closed, he signed for Stevenage Borough after talks with Gillingham stalled.
graysath-onlines sponsored player for the 2008/09 season.