Mossley, Lancashire born left back Billy Dennis played junior football for Ashton PSA and Denton prior to joining the Army during the First World War, during which time he featured for Birkenhead Comets, Linfield and Tranmere Rovers. But after the cessation of hostilities he joined Stalybridge Celtic from where he soon moved to Blackburn Rovers in May 1919, making his Football League debut against Middlesbrough that September. He made 5 first team appearances before returning to Stalybridge Celtic in February 1920, and when Celtic joined The Football League in 1921, Dennis played in their inaugural League fixture against Chesterfield, a match that Celtic won 6-0.
He was a regular over the next two seasons at Bower Fold, making 80 appearances for The Celts, however the club resigned its League status after two seasons, as it was felt that it could not attract enough support to justify a League side in spite of the fact that average attendance of 5,480 in 1922 was almost 2,000 more than neighbouring Rochdale, who played in the same Division.
Dennis joined Second Division Manchester United in May 1923 but he only made 3 appearances in his short spell at Old Trafford before moving on to join Chesterfield in February 1924. He became a regular in the Chesterfield line up over the next four seasons scoring 6 goals, all of which came in 1924-25 and 5 of which were from the penalty spot, in 174 appearances for The Spireites, being ever present in 1926-27 having missed only one game the previous season.
He joined Wigan Borough in May 1928 and had a further two seasons of League football, making 71 appearances, before joining Cheshire League club Macclesfield in the summer of 1930, where he made 45 appearances for The Silkmen. He subsequently played for Hurst from 1931 to 1933, and at the age of 37 joined his hometown club Mossley as trainer, where he completed his football career. He was also a professional cricketer in the Lancashire League.