Kilmarnock, Ayrshire born left back William Agnew began his football career with Afton Lads before joining hometown Scottish League club Kilmarnock in 1900, from where he was signed by Newcastle United in May 1902, making his Football League debut against Stoke that September. An ever present in his first season at St James’ Park, he lost his place in October 1903 before joining Middlesbrough in June 1904 after 44 appearances for The Toon.
At Middlesbrough he was a first team regular and missed only one match during 1905-06, however at the end of the season he left ‘Boro to return to Kilmarnock having scored twice in 72 appearances for ‘Boro, where he played for a further two seasons. During his time at Kilmarnock he was capped three times by Scotland, making his international debut in a 3-0 win over Ireland at Celtic Park in March 1907, winning two further caps in March 1908, playing against Wales and Ireland. He also played twice for The Scottish League against The Football League in March 1907 and February 1908 before being signed by Sunderland in May 1908. In two seasons at Roker Park, he made 28 appearances, becoming only one of four players to play for all three of the largest north east clubs.
He joined Falkirk in September 1910 and stayed three seasons at Brockville Park before a brief spell at Third Lanark in 1913. He then joined East Stirlingshire later in 1913 where he finished his career.