Robertson broke into the Pirates' regular lineup in 1970, playing alongside future Hall-of-Famers Roberto Clemente, Bill Mazeroski and Willie Stargell.

On August 1 of that year, at Atlanta–Fulton County Stadium, Robertson and Stargell each collected five hits in a 20-10 victory over the Atlanta Braves.

Robertson hit two more home runs in the World Series; one of those came in Game Three off Baltimore starter Mike Cuellar with Clemente on second and Stargell on first.

Television replays showed that Clemente had appeared to call time-out just before that pitch; however, Cuellar was already in his windup at the time.

Steve Blass, the winning pitcher in that Game Three, was sitting next to manager Danny Murtaugh in the Pirate dugout.

The pitcher offered to pay the fine if Murtaugh imposed one on Robertson for missing the bunt sign.

Released again, he was signed by the Toronto Blue Jays a few weeks into the 1979 season as a backup for first baseman John Mayberry.