Stockport, Cheshire born goalkeeper William Mellor began his football career with Barrow playing the rugby code, and first played football for North Eastern League club Carlisle United, from where he joined Southern League club Norwich City in 1909, where initially he was understudy to Bobby Beale. Having ascended to first choice at The Nest when Beale moved to Manchester United in 1912, Mellor impressed enough for First Division club Newcastle United to pay £765 for his services in the second week of January 1914, joining as emergency cover and understudy to Jimmy Lawrence. Mellor immediately made his Football League debut in a 3-3 draw with West Bromwich Albion at St James’ Park, playing all 16 matches for The Toon before the end of the season.
He played 8 further matches for Newcastle between Boxing Day 1914 and March 1915 before his career was interrupted by the suspension of peacetime football due to the onset of the First World War in May 1915, but after the War he returned to St James’ Park and made a final appearance in a 2-1 win at Sheffield United in December 1919. He left Newcastle at the end of the season and his career thereafter is uncertain.