A goal-kicking wing, he played rugby league in England for St. Helens, with whom he won the Challenge Cup in 1966.
Killeen also played in Australia for the Balmain, winning the NSWRFL premiership with them in 1969 and becoming the club's all-time top point-scorer.
Len Killeen played on the wing and scored 3-tries and 6-conversions in St. Helens' 35–12 victory over Halifax in the Championship Final during the 1965–66 season at Station Road, Swinton on Saturday 28 May 1966, in front of a crowd of 30,165.
Len Killeen played on the wing scored a try and 5-conversions, and was man of the match winning the Lance Todd Trophy in St. Helens' 21–2 victory over Wigan in the 1966 Challenge Cup Final during the 1965–66 season at Wembley Stadium, London on Saturday 21 May 1966, in front of a crowd of 98,536.
Len Killeen played on the wing and scored a try in St. Helens' 15–4 victory over Leigh in the 1963 Lancashire Cup Final during the 1963–64 season at Station Road, Swinton on Saturday 26 October 1963, and played on the wing and scored 3-conversions in the 12–4 victory over Swinton in the 1964 Lancashire Cup Final during the 1964–65 season at Central Park, Wigan on Saturday 24 October 1964.