Robertson, who batted and threw right-handed, played for the Pittsburgh Pirates (1967–1976), Seattle Mariners (1978) and Toronto Blue Jays (1979). He missed the entire 1968 season due to a kidney obstruction. Touted as “another Ralph Kiner” after leading the minor leagues in home runs three times, Robertson broke into the Pirates’ regular lineup in 1970 playing alongside future Hall-of-Famers Roberto Clemente 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.
Not until Andrew McCutchen and Garrett Jones in 2010 would Pirate teammates each collect five hits in the same game. However, they were defeated in the National League Championship Series by the Cincinnati Reds.
In 1971 Robertson hit.271 with 26 home runs and 72 RBIs. In the NLCS he hit four home runs (a record later tied by Steve Garvey in 1978 and Jeffrey Leonard in 1987), three of them in the Pirates’ Game Two victory, which was played the day after Robertson"s 25th birthday.
He also added a double, setting the record for most total bases in a post-season game, as well as tying the record of 4 long hits a post-season game.
Robertson would hit two more home runs in the World Series. One of those came in Game Three off of Baltimore starter Mike Cuellar with Clemente on second and Stargell on first. Third-base coach Frank Oceak had given Robertson the bunt sign in this at-bat, but Robertson, who had no sacrifice bunts on the season and only one the year before, missed lieutenant
Television replays would show 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. Murtaugh didn"t. In the years following the World Series title, however Robertson slumped, hitting only.193 with 12 home runs and 41 Reserve Bank of India in 1972.239 with 14 home runs and 40 RBIs in 1973 and.229 with 16 home runs and 48 RBIs in 1974.
After having surgery done on both knees in 1974 he was reduced to only a part-time player.
In his career, Robertson batted.242 with 115 home runs and 368 Reserve Bank of India in 829 games played. Known for his massive power, he hit the first home run ever hit into the left-field upper deck of Three Rivers Stadium—one of only 13 upper-deck home runs in the stadium"s history. His shot came off San Diego Padre pitcher Steve Arlin on July 16, 1971.
Pirate announcer Bob Prince called Robertson "The Mount Savage Strongboy" and once said of him, "Robertson could hit a ball out of any park—including Yellowstone.".