Deadmarsh started his professional career in the 1970–71 season for Buffalo's then-farm team, the Salt Lake Golden Eagles of the Western Hockey League (WHL). He made his NHL debut that year, and played a handful of games the following year, while spending most of his time with the Cincinnati Swords of the American Hockey League (AHL), leading his team in goals.[3] After a strong start with the Swords in 1973, he was called up to Buffalo, being traded later that season to the Atlanta Flames for Norm Gratton.
However, the Scouts sold his rights later that year to the Vancouver Blazers of the World Hockey Association (WHA), the first-ever such transaction, after Deadmarsh demanded to be dealt there, having signed a contract with the Blazers to take place after the 1974–75 season.[6][7] The Blazers relocated to Calgary the following year, and Deadmarsh had his best professional season, finishing fourth in team scoring with 26 goals and 28 assists, as well as leading the Cowboys in penalty minutes with 196.[8]
In his NHL career, Deadmarsh played in 137 games, scoring twelve goals and adding five assists. He played in 255 WHA games, scoring 63 goals and adding 66 assists. He is a second cousin of former NHL player Adam Deadmarsh.