The Pittsburgh Pirates finished a three-game sweep of the New York Mets with a 2-1 victory yesterday at Pittsburgh to clinch the National League East Division Championship.

The victory gave the Pirates an insurmountable 41/2-game lead over the second-place Chicago Cubs, who beat Philadelphia, 5-3, yesterday on the strength of Ferguson Jenkins' four-hitter.

Playoffs Saturday

Pittsburgh begins the National League playoffs against the Cincinnati Reds, the West Division champions, at Three Rivers Stadium next Saturday.

Second baseman Dave Cash scored one run and batted in another to give Pirate pitcher Dock Ellis the victory. Donn Clendennon knocked in the only run for the Mets.

Football

At Harvard Stadium, the New York Jets capitalized on four miscues by the Boston Patriots and coasted to a 31-21 victory yesterday.