After his second Stanley Cup success, Mackell was traded by Toronto to the Boston Bruins in return for young defenceman Jim Morrison.

In Boston, he attained NHL First All-Star Team status for the 1952–53 season, at the position of center, playing on a line composed of Ed Sanford and Johnny Peirson.

Mackell also won the Elizabeth C. Dufresne Trophy in 1953, awarded annually to the outstanding Bruins player on Boston Gardens home ice.

In 1956–57, Mackell centered a line composed of veteran iron man Cal Gardner and Rookie Of the year, Calder trophy winner Larry Regan.

The versatile Mackell frequently played at left wing, principally with Don McKenney and Jerry Toppazzini, a trio whose production matched the output of the vaunted UKE Line, along with his regular position at centre.