Reilly played for Castleford Tigers in England and Manly-Warringah Sea Eagles in Australia. After the 1969 Challenge Cup final he was awarded the Lance Todd Trophy for his man-of-the-match performance. He later played in Australia, helping the Manly-Warringah Sea Eagles to their first premiership in 1972 and to their second the following year.
After retiring from playing, Reilly went on to coach Halifax RLFC and Great Britain. He opened his reign as head coach of Great Britain with six wins, four against France and two over Papua, in 1987. He stood down as Great Britain's coach in 1994. Reilly then took over Australian side Newcastle Knights in 1995 as coach and their results improved markedly. He later coached them to their 1997 ARL Grand Final success.
In 2001 Reilly's book, Reilly: A Life in Rugby League was published. His coaching career continued but he had less success as head coach of the Huddersfield Giants, who finished bottom of the Super League under his stewardship. He left Leeds at the end of the 2003 season following a major coaching re-structuring. In December 2004, Hull Kingston Rovers appointed Mal Reilly as director of rugby and first team coach. However, Reilly left the club mid way through the season.