Sullivan was one of the tallest pitchers of his time, standing 6 feet 6 inches (1.98 m) tall. After the 1960 season, the Red Sox traded him to the Phillies for another towering right-hander, 6 ft 8 in (2.03 m)-tall Gene Conley. Coincidentally, Conley had been the winning pitcher and Sullivan the loser of the 1955 All-Star Game. A walk-off home run by Stan Musial on the first pitch from Sullivan in the bottom of the 12th inning brought the midsummer classic to an abrupt end. Sullivan had entered the game with two men out in the eighth and had held the National League (NL) scoreless for 31⁄3 innings prior to Musial’s clout.
In 1955, Sullivan topped the AL with 260 innings pitched and tied with Whitey Ford for the most wins (18). For his career, he posted a 97–100 win–loss record, with a 3.60 earned run average (ERA), in 351 pitching appearances. He dropped 18 of his 21 National League decisions as a member of the Phillies, but went 94–82 in the American League. Overall, Sullivan permitted 1,702 hits and 559 bases on balls in 1,732 MLB innings pitched. He struck out 959.
In September 2008, Sullivan published a memoir entitled, Life Is More Than 9 Innings.