Ballymena bounced back from Friday's defeat at Glenavon by narrowly beating Institute to go top of the table.
They had to battle back from an early setback as Stephen O'Flynn's fine start to the season continued as he scored a close-range opener in the third minute.
Ally Teggart got Glenn Ferguson's Sky Blues level before the break with a sweetly-struck effort.
Teggart got the winner with a 65th-minute penalty after Stephen McBride had been fouled in the area.
With Cliftonville being held at home by Linfield, the hard-fought win took United back to the top of the table with 13 points from six matches.
It is an encouraging start to the campaign for Ballymena who have not finished in the top half of the Premiership since the end-of-season split was introduced in 2008-09.
"We actually played better on Friday night and lost," said Ballymena manager Ferguson.
"So it is great to have played not so well and still won the game.
"But you have to give a lot of credit to Institute because they came here and tried to play football.
"That is refreshing because we have played matches where there has been a lot of route one football and we have had to battle for points.
"It is good to get the ball on the deck and both teams tried to play football."