Forsyth began his football career in 1967 as a member of the
Arsenal ground staff. However, he was released at the end of the season and returned to Scotland, where he signed for
Partick Thistle. In 1971, Forsyth was a member of the Thistle team that won both the
Scottish First Division and the
Scottish League Cup, beating
Celtic 4–1 in the
final on 23 October 1971. His performances during the 1971–72 season earned him a selection for the
Scottish League XI for their game against
The Football League XI on 15 March 1972, and also attracted the eye of
Scotland manager
Tommy Docherty, who gave Forsyth his first cap against
Yugoslavia on 29 June 1972. He continued in the Scotland team for three more matches that year, ultimately earning a total of 10 caps. He also played once for the
Scotland Under-23s against
Wales Under-23s in February 1974. When
Manchester United hired Docherty to replace
Frank O'Farrell as their manager in December 1972, Forsyth was one of several Scottish players signed by Docherty, costing the club £100,000. He made his debut at left-back in a 3–1 defeat away to his former club, Arsenal, on 6 January 1973. He then played in a further eight consecutive matches – including a 1–0 away defeat to
Wolverhampton Wanderers in the third round of the FA Cup – before losing his place to
Steve James following a 3–1 loss to
Birmingham City on 10 March. Forsyth made only sporadic appearances in the first half of the 1973–74 season, but became a regular in the team from the middle of January 1974, mostly playing at right-back. He scored his first goal for the club in a 3–3 home draw with
Burnley on 3 April 1974, but he was unable to prevent the team from finishing second from the bottom of the
First Division and suffering relegation to the
Second Division. He continued as a regular in the first team in 1974–75, missing only six matches all season as the team won the Second Division and regained their place in the top flight, as well as reaching the semi-finals of the
League Cup. His only goal of the season came in a 3–0 away win over
Blackpool on 19 October. The emergence of
Jimmy Nicholl at right-back in 1975–76 meant that Forsyth missed all of September, October and November 1975, but he regained his place in the team from December onwards, playing in every game until the end of the season, including Manchester United's run to the
1976 FA Cup Final. Despite this consistent run in the team, Forsyth was dropped to the reserves in place of Nicholl for the start of the 1976–77 season, and he made just five appearances all season, including a three-game spell between 27 November and 18 December 1976. He then came on as a substitute in a 4–0 away defeat to
Queens Park Rangers on 19 April 1977 before finishing his season by starting in a 1–0 away defeat to Liverpool on 3 May. He made only four appearances in 1977–78, and at the end of the season, he was allowed to join
Rangers, initially on loan before the transfer was made permanent in August 1979. He later played for
Motherwell,
Hamilton Academical and
Queen of the South, before ending his career with
Scottish Junior Football Association club
Blantyre Victoria. ==Honours==