Billy Mercer won a cap for England while at St. Helens in the 19–35 defeat by Other Nationalities at Thrum Hall, Halifax on Monday 7 April 1930.

[2] Bill Mercer made his début for Lancashire in the 15–7 victory over Cumberland at Whitehaven on Tuesday 29 October 1929, alongside fellow St. Helens player Alf Ellaby who scored three tries, Mercer had two further victories alongside Alf Ellaby, against Yorkshire and Glamorgan and Monmouthshire as Lancashire won the County Championship.

Bill Mercer played at centre in St. Helens' 9–5 victory over Huddersfield in the Championship Final during the 1931–32 season at Belle Vue, Wakefield on Saturday 7 May 1932.

Bill Mercer played at centre in St. Helens' 3–10 defeat by Widnes in the 1929–30 Challenge Cup Final at Wembley Stadium, London on Saturday 3 May 1930, in front of a crowd of 36,544.

Bill Mercer made his début for St. Helens in the 15–3 victory over Batley at Knowsley Road on Saturday 26 September 1925, and scored his first try in the 55–6 victory over Bradford Northern at Odsal Stadium on Wednesday 14 April 1926, his last match was the 7–25 defeat by Warrington at Knowsley Road on Saturday 6 November 1937.