"We won the game, didn't we," the St Louis manager, Tony La Russa, said. "He was on the winning team. He'll probably get the record, don't you think?"
Ray Lankford hit a two-run homer in the fifth innings, which was capped by the rookie J D Drew's pinch two-run single to help St Louis win for the ninth time in 11 games.
In Chicago, McGwire's main rival Sammy Sosa remained stuck on 58 home runs for the season, but the Chicago Cubs kept pace in the wild-card race with a 5-2 victory over the Pittsburgh Pirates.
The high-flying New York Yankees clinched the American League Eastern Division title in just their 143rd game as Derek Jeter and Paul O'Neill each hit two homers in a 7-5 victory over the Boston Red Sox.
Their run made them the second fastest side to clinch the title in the 29-year history of divisional play, coming one day later than the 1975 Cincinnati Reds. "We worked hard all year and we've had a great year." said Jeter. "We hadn't accomplished anything until now. Everybody anticipated us winning the division. You never know what may happen, so we wanted to put it away early."
The Yankees improved baseball's best record to 102 wins and 41 losses while wrapping up their seventh divisional title since 1975 and second in three years. The Yankees are involved in post-season play for the fourth straight year after securing the American League wild card in 1995 and 1997.
Their pitcher Ramiro Mendoza earned the win, allowing one run and three hits over three innings in relief of Orlando Hernandez.