Rick Mahler, the winningest pitcher in the majors, improved his record to 7-0 and Terry Harper drove in three runs in the Braves' 6-1 victory over the Montreal Expos yesterday in Atlanta.

Mahler is the only starting pitcher to win a game this season for the Braves (11-12). He gave up eight hits in eight innings, allowing Dan Driessen's fourth home run of the season in the second inning. Bruce Sutter pitched the ninth for Atlanta.

"There were times when I thought my opportunity would be with another organization," said Mahler, who was demoted to the minors in 1983 and was 13-10 last season with the Braves. "I'm just glad with the way things turned out.

"I hope it doesn't end. I know it has to sometime, but for now I don't even want to think about that. I don't know why this club has been playing better behind me than behind our other starters. We've got some good pitchers and we should have a better record than we do."

Mahler's offspeed pitches particularly baffled the Expos. "I've never seen a change-up that slow," second baseman U.L. Washington said.

The Braves scored twice in the first off Bill Gullickson (3-3). Gerald Perry hit a two-out double and scored when third baseman Tim Wallach overthrew first base on a single by Dale Murphy. Murphy scored later on a single by Harper.

Rafael Ramirez doubled in Bruce Benedict in the second to put the Braves ahead, 3-1. They made it 5-1 in the fifth.