Born in Platt Bridge, Wigan, right half Jack Jennings played for junior club Platt Bridge Wesleyans from 1920 before beginning his senior football career with Wigan Borough in May 1923, making his Football League debut against New Brighton in January 1924, soon establishing himself in Borough’s first eleven. He joined First Division Cardiff City in May 1925 after a single goal in 48 appearances for Wigan, making his debut in a 3-2 defeat to Birmingham City on 10th October 1925. However, due to competition from several established players including Billy Hardy and Harry Wake, he made just one further appearance during the 1925-26 season in a 0-0 draw with Arsenal on 27th February 1926.
The following season, Jennings did not make an appearance until the final three months of the season, being handed a chance at full back following an injury to Tom Watson, but impressed enough to eventually displace Watson from the side. He became a permanent fixture in the first team at Ninian Park, featuring in every League match that the club played for nearly two years between February 1928 and January 1930, before he was sold to Middlesbrough as part of a triple transfer that included Joe Hillier and Fred Warren after 97 appearances for The Bluebirds.
Jennings spent several years as club captain at Middlesbrough, eventually relinquishing the role to Tom Griffiths following his arrival from Bolton Wanderers in 1933, and was selected in an FA XI for the Tour of Canada in 1931. After 10 goals in 205 appearances for Middlesbrough he joined Preston North End in September 1936, staying only 6 months at Deepdale, scoring once in 20 appearances, before he signed for Bradford City in March 1937. He made 34 appearances for The Bantams and then hung up his boots in the summer of 1938.
After the Second World War he became a trainer at Northampton Town in 1945. He later worked as a coach for the England national amateur football team and the Great Britain Olympic football team, working as a trainer for Norman Creek, Charles Hughes and Walter Winterbottom between 1952 and 1971. He also worked as a trainer for player-manager Jimmy Armfield on an FA XI tour of Tahiti, New Zealand, Singapore, Hong Kong and Thailand in 1969.