Standing 6'5", Loscutoff was selected with the third non-territorial pick of the first round in the 1955 NBA draft. In nine seasons from 1955 to 1964, he played forward and won seven championships as part of the legendary Celtics teams of the 1960s. He was originally drafted by coach Red Auerbach to provide some much-needed defensive nerve for the Celtics team, which (despite becoming the first team to average 100 points per game in the 1954–55 season) had one of the worst defensive records in the league.
Loscutoff was described by one player as a hatchet-man, and his defense and strength were part of the defensive greatness of the 60s Celtics, alongside Hall-of-FamerBill Russell.
Loscutoff's nicknames included "Jungle Jim" or "Loscy". The organization wished to honor Loscutoff, but he asked that his jersey number (18) not be retired, so that a future Celtic could wear it. Instead, the Celtics added a banner with his nickname "Loscy" to the retired number banners hanging from the rafters of their arenas. The number was later retired in honor of another Celtic great, Dave Cowens.