McNally has the unique distinction as the only pitcher in Major League history to hit a grand slam in a World Series game (Game 3,1970, a 9–3 victory). The bat (lent to him by teammate Curt Motton) and ball are in the National Baseball Hall of Fame Museum in Cooperstown, NY.
McNally is also part of World Series history for his (and his pitching mates') performance in the 1966 World Series, which the Orioles swept over the defending champion Los Angeles Dodgers. In the fourth game, he and Don Drysdale matched four-hitters; one of Baltimore's hits was Frank Robinson's fourth-inning home run for a 1–0 Oriole victory. McNally's shutout capped a World Series in which Baltimore pitchers set a Fall Classic record by pitching 33⅓ consecutive shutout innings, beginning with Moe Drabowsky's 6⅓ scoreless innings in relief of McNally in Game One, followed by shutouts from Jim Palmer and Wally Bunker. Ironically, the trio had pitched one shutout total during the regular season—that by McNally on August 6 against the Washington Senators.
He won more than 20 games for 4 consecutive seasons (1968 through 1971) and was one of four 20-game winners for the 1971 Orioles (Pat Dobson, Jim Palmer, and Mike Cuellar were the other three). He was the only pitcher other than Roger Clemens to win 12 decisions in a row 3 times, including 17 consecutive at one time. After winning the last 2 decisions of the 1968 season, he opened the 1969 season with a 15–0 record.
On September 28, 1974, McNally gave up Al Kaline's 3,000th career hit.
He is also known for his role in the historic 1975Seitz decision which led to the downfall of major league baseball's reserve clause and ushered in the current era of free agency. McNally and Andy Messersmith were the only two players in 1975 playing on the one year reserve clause in effect at the time. Neither had signed a contract at the time but both were held with their team under the rule. The two challenged the rule and won their free agency.
McNally retired after the 1975 season and had no intention of claiming his free agency. According to John Helyar's book The Lords of the Realm, players union executive director Marvin Miller called McNally---technically still an unsigned player---to ask him to add his name to the grievance it had filed in opposition to the reserve clause and he agreed. The reason Miller thought of McNally, Helyar wrote, was "insurance" in the grievance in the event Andy Messersmith decided to sign a new contract after all. Baseball owners wanted his name off the grievance so the Expos offered McNally a $25,000 ($109,570 today) signing bonus and a $125,000 ($547,851 today) contract if he made the team, but McNally declined. The hope was to sign Messersmith at the same time, thus eliminating the challenge.
In an article in 1976 in Esquire magazine, sportswriter Harry Stein published an "All Time All-Star Argument Starter", consisting of five ethnic baseball teams. Because of space limitations the Irish team, including McNally as left-handed pitcher, was omitted.