Armadale, Lothian born left half Jimmy Forsyth began his football career with Armadale and played for Bathgate before heading south to join Second Division club Portsmouth in 1925, making his Football League debut against Blackpool that October. After 3 matches for Pompey in his debut season he remained in the reserves for two further seasons before joining Gillingham in June 1928, where he played 36 matches as Gillingham finished towards the bottom of the Third Division (South).
Forsyth signed for Second Division club Millwall in May 1929 and in January 1930 first appeared at inside left, playing there for most of 1930-31 and for some of 1931-32, scoring 11 goals in the former. He then scored 15 goals in 1932-33, his career best, including scoring a brace in 5 different matches, but he reverted to left half from September 1933 and although Millwall were relegated from the Second Division that season, Forsyth remained with the club and missed only two matches in their 1935-36 campaign.
Forsyth was a regular in their 1936-37 campaign which featured an FA Cup run that saw Millwall reach the FA Cup semi final, having beaten the likes of Fulham, Derby County and Manchester City, before losing to eventual winners Sunderland at Leeds Road, Huddersfield. They followed up this success by winning the Third Division (South) Championship in 1937-38, when Forsyth missed only 5 matches. Back in the Second Division, Forsyth missed only one match before the outbreak of the Second World War in September 1939 effectively finished his football career after 49 goals in 345 appearances for Millwall.
Forsyth finally retired from playing in 1943 to become assistant trainer of Millwall. He then became first team trainer at Ipswich Town, a post he held for 15 years, and was caretaker manager of the club for a brief period in September and October 1964, between the departure of Jackie Milburn and the arrival of Bill McGarry, taking charge of 7 matches. He retired in 1971.