Manchester born wing half Jack Brennan began his football career with Ancoats Lads Club in 1908 and played for Hollinwood United in 1909 before having an unsuccessful trial with Manchester City in March 1911. He joined Denton later the same year before joining Second Division Glossop, initially as an amateur, in 1912, making his Football League debut against Lincoln City that October, making 20 appearances in his debut season, mainly at centre half. Having played just once more for Glossop in November 1913, he was signed later the same month by First Division Bradford City, where he appeared 11 times for The Bantams during the remainder of 1913-14.
He joined hometown club Manchester City in July 1914 and made 35 appearances during the final season prior to the suspension of peacetime football due to the onset of the First World War. during which he served in the armed forces, and when peacetime football resumed in 1919 he returned to Hyde Road, but lost his place in September 1919, before being recalled to the City first team in January 1920, eventually making 22 appearances in the season. However he made only two appearances during 1920-21 as City finished runners up in the League Championship, and after his final appearance for The Citizens at Tottenham Hotspur in February 1922 he was released to join Rochdale in July 1922 after 60 appearances for Manchester City. However he failed to make the first team at Spotland and retired in 1923.