He was sold on to Derby County for a £40,000 fee in June 1982 before moving on to Mansfield Town the following year. He spent the next decade with the "Stags" and, after being named as the club's Player of the Year, was then named on the PFA Team of the Year as he helped Mansfield to win promotion out of the Fourth Division in 1985–86. He also won the Football League Trophy with the club in 1987.
He was appointed player-manager in February 1989 and led the club to promotion out of the Fourth Division in 1991–92, but was sacked after failing to keep Mansfield in the Third Division. He ended his playing career with 649 league appearances in a 20-year career in the Football League.[5] He later managed Conference club Telford United from 1993 to 1995, before becoming a professional scout.
Argyle went on to finish 16th in the Second Division in 1975–76 before slipping back into the third tier after finishing two points short of safety in 1976–77. Foster also spent a brief time on loan at Devon rivals Torquay United in October 1976, scoring three goals in six league games. The "Pilgrims" replaced managerTony Waiters with Mike Kelly, and only avoided a second-successive relegation by just two places and three points in 1977–78 after Malcolm Allison replaced Kelly as manager in March. Foster was changed to the centre-back position during the campaign, and it was there that he established himself as a mainstay in the starting eleven.[2]
Under the stewardship of new boss Bobby Saxton, Plymouth moved up to a more secure 15th-place finish in 1978–79, and Foster recovered from a broken leg to regain his first-team spot for the 1979–80 campaign.[2] Plymouth rose to seventh place in 1980–81, before new manager Bobby Moncur led the club to a tenth-place finish in 1981–82. Foster spent part of the season on loan at Brian Godfrey's Exeter City, playing 28 Third Division games at St James Park after making his debut against Plymouth on 28 December.[3]
He was an ever-present at Exeter for the remainder of the campaign and became the first player to win the club's Player of the Year award whilst on loan from another club.[3] He turned down a transfer to Sheffield United as he did not want to drop down to the Fourth Division.[7] During eight years at Home Park, Foster scored six goals in 248 league and cup appearances and was named as the club's Player of the Year in 1978 and 1980.
Foster joined Mansfield Town on a free transfer in the summer of 1983. He made his Mansfield debut on 27 August 1983 in a 4–0 defeat against Bristol City.[8] However, things would soon take a turn for the better, and Foster was named the club's Player of the Year as Ian Greaves's "Stags" finished 19th in the Fourth Division in 1983–84. He was then named team captain and marshalled a defence that only conceded 38 goals in the league in 1984–85. In 1985–86, Foster helped the Field Mill club gain promotion to third place and was recognised by his peers with a place on the PFA Team of the Year.
When Ian Greaves stepped down as Mansfield manager in February 1989, Foster was named player-manager. The "Stags" ended the 1988–89 season in 15th place in the Third Division. In his first full season in charge, 1989–90, Mansfield again posted a 15th-place finish. After finishing bottom of the division in the 1990–91 campaign, they suffered relegation. Foster then took the Field Mill club straight back up after leading the club to the third and final automatic Fourth Division promotion place in 1991–92. However, he could not keep the club in the Third Division, as they went back down in 1992–93. In September 1993, following a poor start to the 1993–94 season, Foster was sacked as manager. Mansfield went on to finish in 12th place under new manager Andy King.
Telford United
In October 1993, Foster was named player-manager at Telford United, where he stayed until June 1995. He led the "Bucks" to 17th and 19th-place finishes in the Conference in 1993–94 and 1994–95, as Telford finished just above the relegation zone.