Mackell began his National Hockey League career with the Toronto Maple Leafs in 1948. The majority of his career was spent with the Boston Bruins where he would retire following the 1960 season. MacKell won two Stanley Cups with the Maple Leafs in 1949 and 1951. He is the son of former Ottawa Senator Jack MacKell. He played junior hockey with the St. Michaels College Majors in 1946-47. Mackell won the Eddie Powers Memorial Trophy as the leading scorer and the team won the Memorial Cup that year.