The St. Louis Cardinals clinched the National League's Eastern Division tonight with a 4-2 victory over the Montreal Expos, last year's East winners.

"This is like winning the first leg of the Triple Crown," said Jim Kaat, the Cardinals' 43-year-old relief pitcher. "We've just won the Kentucky Derby, and now there's two big races to go."

The victory, making Manager Whitey Herzog's team the first team to clinch its division this year, also gave the Cardinals their first title since the advent of divisional play in 1969. Their last pennant was in '68.

"Two weeks ago, it looked like it was going to go down to the wire," said Herzog as he and other members of the team toasted themselves with champagne in the locker room. "But we put it on when we had to. We were able to avoid a losing streak and that was a big plus for us."

St. Louis will be host for the first two games of the NL playoffs Oct. 6 and 7, with the rest of the best-of-five series in the park of the West Division champion Oct. 8, 9 and 10.

Tonight, Willie McGee's three-run, inside-the-park homer highlighted a four-run first inning. Dave LaPoint (9-3) was the winning pitcher.

After a rain delay of 63 minutes, Ken Oberkfell hit a one-out double in the first inning and scored on a single by Dane Iorg. After loser Bill Gullickson (12-13) walked Darrell Porter, McGee hit a line drive to left center that Expos center fielder Andre Dawson missed on an attempted shoestring catch. "I didn't make the play. That's all that counts," said Dawson.

By the time the ball was relayed home, McGee slid in safely for his fourth homer of the year to make it 4-0.

Montreal stranded 10 runners in the first six innings, including three in the sixth.

LaPoint went 5 2/3 innings and gave up six hits. Bruce Sutter got his 36th save.