He started out in junior hockey with the Verdun Maple Leafs and then moved on to the Kitchener Rangers and the Peterborough Petes. When he finished with junior he was sold by the Montreal Canadiens to the Minnesota North Stars in 1968. Rathwell turned pro with the Clinton Comets of the Eastern Hockey League in 1968-69, and helped his team win two straight league titles.
The California Golden Seals accquired him in 1970 and assigned Rathwell to their Salt Lake Golden Eagles farm team. In 1973, after some more trades, Rathwell helped the Cincinnati Swords win the Calder Cup.
Finally in 1974-75, while playing for the Rochester Americans, Rathwell got an emergency callup to the Boston Bruins and played an NHL game. At the end of that season, he retired in order to provide a more stable enviroment for his family.