Born in Platt Bridge, Jennings had worked as a railway fireman before beginning his footballing career with Wigan Borough.
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 31 February 1926.
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.
[3] Jennings spent several years as club captain at Middlesbrough, eventually relinquishing the role to Tom Griffiths following his arrival from Bolton Wanderers in 1933,[3] and was selected in an FA XI for a tour of Canada in 1931.
[8][9] 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.