Born in Billings, Montana, McNally was raised by his mother after his father died in the Battle of Okinawa. He was signed by the Orioles out of high school in 1960 and made his major league debut two years later, throwing a shutout in his first game in the major leagues. From 1963 through 1965, he continued to refine his pitches while securing his grip on a starting spot in the Orioles' rotation. In 1966, he made two starts in the World Series, the second of which was a shutout that gave the Orioles a 4–0 victory over the Los Angeles Dodgers. McNally was bothered by a calcium deposit in his elbow in 1967; the injury limited his playing time in the second half of the season. He was named the Comeback Player of the Year in 1968 as he finished second in the American League with 22 wins.