Born in Lindsay, Ontario, MacLeish spent the majority of his junior career with the OHA's Peterborough Petes. The Boston Bruins took him fourth overall in 1970 after notching 101 points and earning a place on the OHA first all-star team. After starting his pro career with the CHL's Oklahoma City Blazers, the young pivot was traded to the Philadelphia Flyers in February, 1971. He was promptly given a chance to get his feet wet in the NHL and looked fairly solid in 26 games.

The Flyers opted to start MacLeish in the AHL in 1971-72 before recalling him late in the year. The next season he broke through with 50 goals while playing the power play and taking a regular shift with Gary Dornhoefer and Ross Lonsberry. Over the next two season he continued to produce on offense but was asked to take on penalty killing and defensive responsibility. MacLeish's excellence was a key component on the Flyers' consecutive Stanley Cup wins in 1974 and 1975. During the first title run he led all playoff goal scorers with 13 goals and 22 points. Overall, MacLeish was named to play in the 1976, 1977 and 1980 NHL all-star games.

During the late 70s, MacLeish was a top two-way player for Philly. In 1979-80 he scored 31 goals while teaming with Bob Kelly and Dennis Vervegaert. The trio helped the Flyers set an NHL record by going undefeated in 35 straight games and later reach the 1980 Stanley Cup final. In July 1981, the veteran pivot joined the youthful Hartford Whalers. During his last three NHL seasons MacLeish also played in Pittsburgh, did a second tour in Philly and dressed for the Detroit Red Wings before retiring in 1984.