Dwayne Nelson was the Ballymena hero as the injured keeper pulled off a superb late save to deny Glentoran.
The Glens failed to turn possession into chances and United's Gary Liggett went closest when his long-range shot was tipped over by Aaron Hogg.
Nelson was injured in a collision just after the break but he remained on as the hosts had no substitute keepers.
Ballymena forward David Cushley was sent-off before Nelson kept out an Andy Waterworth strike on 88 minutes.
It was a game of few chances on a heavy Showgrounds pitch as the Glens looked to bounce back from Monday's 5-0 hammering by Sligo Rovers in the Setanta Cup.
Glens midfielder Jim O'Hanlon found space but opted against taking a shot and his low cross failed to find a connection.
United were dangerous on the break and Hogg saved from Liggett and Tony Kane while Allan Jenkins volleyed wide.
Nelson battled on despite his obvious discomfort and he secured a point for the Sky Blues with the excellent late save.