After a stellar junior career with the Peterborough Petes and representing Canada at the 1980 World Junior Ice Hockey Championships, Murphy was drafted 4th overall in the 1980 NHL Entry Draft by the Los Angeles Kings. In the 1980–81 NHL season, he set National Hockey League records for most assists and points by a rookie defenceman, with 60 and 76 respectively. He was eventually traded to the Washington Capitals for defenceman Brian Engblom and winger Ken Houston early in the 1983–84 season, which did not work out in the Kings' favor long-term because of Murphy's prolonged success in the league.