Born in Copper Cliff, Ontario, Sleaver left for Galt, Ontario in 1950 to play for the Chicago Black Hawks OHA affiliate. He would play five seasons in Galt, including on teams featuring young Kenny Wharram and Bobby Hull.
In the 1953-54 season Sleaver had his first taste of NHL action. He was called up for one game in which he made the scoresheet. Unfortunately it was for taking a two minute penalty.
It took two more years before Sleaver got an extended look. In a 12 game stretch he score his only NHL goal and point.
From that point on John was relegated to a long vagabond career in the minor leagues, playing for 11 teams in five different leagues.
Sleaver's longest minor league stay was in Rhode Island, and he made Smithfield his after hockey home and was very active with the Lions Club. He later retired to Suzy Lake, Florida where he passed away in 2001.