Tyldesley, Lancashire born outside right Peter Cowper began his football career playing for Burns Celtic towards the end of the First World War and played for Parkside Rangers in 1919, Atherton Collieries in 1920 and Rossendale United in 1923, having unsuccessful trials with Bolton Wanderers and Wigan Borough before joining First Division club West Ham United in June 1924, where he made his Football League debut against Bury in October that year, also playing a week later in a defeat at Nottingham Forest.
Cowper remained in the reserves at West Ham, where he had a reputation for his willingness to play anywhere, but didn’t get any further first team opportunities before joining Second Division club Grimsby Town in June 1927, where he was again principally a reserve team player, making 4 first team appearances in the late summer of 1927, but he soon joined non league Lancaster Town before returning to Football League action with New Brighton the following August. It was at Sandheys Park that he had easily the most impact, missing only 2 matches in his debut season, scoring 16 goals for New Brighton, finishing as their top goalscorer for 1928-29, and although the goals dried up the following season, he did enough to convince Second Division Southampton to sign him in June 1930 after 19 goals in 72 appearances for The Rakers.
Cowper made his debut for The Saints when he took the place of Irish international Laurie Cumming at inside right in a 3-0 home defeat by Tottenham Hotspur on Boxing Day 1930. Cumming returned for the next match but later in the season Cowper made three appearances at outside right in place of Bert Jepson, followed by a single first team appearance in February 1932, although he also scored 33 goals in 50 matches for Southampton reserves.
He was then signed by Third Division (North) club Southport in March 1932 and made a pair of appearances for The Sandgrounders the same month, joining Carlisle United in July. He scored once for Carlisle in 13 appearances during 1932-33 before a move to Cheshire League club Wigan Athletic, where he helped them to win the League Championship in 1933-34 scoring 13 goals in 37 appearances. He then joined Altrincham in the summer of 1934, where he stayed two seasons before a move to Prescot Cables, his final club before his eventual retirement.