The Billings Gazette reported on its website that McNally had been battling cancer for years.
The soft-tossing lefthander played 14 seasons in the majors, all but one with the Orioles, in a career that began in 1962. He won 20 games each year from 1968-71.
In 1971, he was one of four 20-game winners on the Baltimore staff, joining Mike Cuellar, Jim Palmer, and Pat Dobson.
McNally posted a career of 184-119 and a 3.24 ERA while striking out 1,512 batters. He played in four World Series, and is the only pitcher to hit a grand slam in the Fall Classic, connecting in Game Three of the 1970 World Series against the Cincinnati Reds.
McNally, along with fellow pitcher Andy Messersmith, also was part of a landmark legal case that challenged baseball's reserve clause. He filed a class-action lawsuit claiming that the Montreal Expos had reneged on a verbal agreement, and the case created free agency.
The resulting labor deal meant players could become free agents after they had played in the major leagues for six seasons. At the beginning of the 1976 campaign, average individual salaries rose to $2.38 million.
He finished his career with the Expos in 1975.
After he retired, he became a businessman in Billings, running an auto dealership with his brother, Jim. He was named Montana's Athlete of the Century in 2000, and also is a member of the Orioles' Hall of Fame and All-Century team.