The two-time Ashes winner, who played 63 Tests and 58 one-day internationals for his country, will take charge of his hometown club, which sits in the ninth tier of English football, alongside head coach Lee Skinner and Lee Anderson.
Chairman Ian Lavery told the club's official website: "All three have a wealth of experience in the sporting world and together can build on the success that Ashington Football Club have had since moving to Woodhorn Lane."
Harmison played in central defence for the club until his late teens before giving up football to concentrate on cricket, from which he announced his retirement in 2013.
The 36 year old takes charge with the club sitting in 17th out of 22 teams after a 4-0 defeat to Marske United on the weekend.
His first match will see him travel to Bishop Auckland in midweek.
Harmison is not the only member of the 2005 Ashes winning team to branch out into a new sport after retiring from cricket.
Close friend Andrew Flintoff tried his hand at boxing after initially hanging up his whites in 2010, winning a four-round decision over little-known heavyweight Richard Dawson in his only professional bout in November 2012.
Flintoff has since made a return to Twenty20 cricket with Lancashire and BBL side Brisbane Heat.
