Billinge-born Greenall made his professional debut with Blackpool on 23 September 1980, at the age of 16 years and 237 days, becoming the club's then-youngest-ever league player (a record broken by Trevor Sinclair in 1989).
Greenall was one of a host of young players brought to Blackpool by Alan Ball during his short managerial reign at the club.
He eventually took over the central role, combining with captain Steve Hetzke and Mike Conroy to form a successful defensive partnership.
Greenall joined Oxford United in February 1988 for £235,000, and the U's soon made him skipper following the departure of Tommy Caton, as he enjoyed a brief taste of top-flight football before they were relegated at the end of the season.
After coming out of retirement, Greenall made his Wembley debut in 1999 when he defied his advancing years to help Wigan beat Millwall 1–0 to win the Football League Trophy.