Fenton, Stoke-on-Trent born left back Len Birks began his football career in local junior football with Butt Lane Star in 1914 but after the First World War he joined Second Division Port Vale in May 1920, making his Football League debut at Blackpool in March 1921. A fringe player in 1921-22, he established himself from the start of the next season, missing only one game during 1922-23 and being an ever present in 1923-24. His consistency brought him to the attention of Sheffield United, who signed him for “a substantial sum” in October 1924 as the Blades sought a replacement for regular left back Ernest Milton. Birks initially played 21 League games in his debut season for The Blades, however he only played in one of their FA Cup ties and missed out on a place in the 1925 FA Cup Final with the ageing Milton being preferred.
He was a near ever present for United in 1927-28, missing one game and part of their 1928 FA Cup semi final team beaten 1-0 in a second replay by Huddersfield Town at Maine Road, Manchester. He also missed only one game in 1929-30, but age was catching up with him and, already aged 34, he joined Second Division Plymouth Argyle in February 1931 after 211 appearances for Sheffield United, competing for the left back spot at Home Park with Fred Titmuss for the next two and a half seasons, during which he made 50 appearances for The Pilgrims. In the 1933 close season he joined Bristol City, having a season at Ashton Gate, making 34 appearances for The Robins, before joining non league Yeovil & Petters United in 1934 prior to his eventual retirement.