Shay McCartan bagged a second-half double to put a huge dent in Glentoran's title hopes at the Warden Street Showgrounds.
Mick McDermott's side looked to be on course for a valuable three points having taken a two-goal lead through Seanan Clucas' brace - his first goals of the season.
Gutsy Ballymena United hit back with McCartan displaying his predatory instincts to help his team pinch a point. The result keeps Glentoran in third place in the Danske Bank Premiership table.
Glentoran should really been in front within 30 seconds. Marcus Kane produced a long throw in that was flicked on by Ruaidhri Donnelly for the usually lethal Robbie McDaid, who completely mis-kicked with only young goalkeeper Dylan Graham to beat.
They threatened again in their next attack with Gael Bigirimana blasting a 25-yard free kick over the crossbar after Jay Donnelly had been flattened by Tony Kane.
The Glens took the lead on 14 minutes and it was just the start Graham didn't want. Clucas managed to trick his way past Kane, who was clearly struggling from an early knock, and his weak shot trickled under the legs of the debutant.
United attempted to respond with Ross Redman whipping in a free kick from the left, which ws only half cleared by Paddy McClean and Ryan Waide could only hoof his shot over the top.
United had big appeals for a penalty on 54 minutes when Waide went down under a challenge from McClean inside the box, but referee Steven Gregg wasn't impressed.
The action quickly switched to the other end with United skipper Jim Ervin rescuing his team with a goal line clearance from Jay Donnelly, following a pin-point cross from Caolan Marron.
The Glens had another chance on 63 minutes when Ervin's botched clearance fell for McDaid, but his scuffed shot was flicked off the line by Jonny Addis.
Clucas struck again with 16 minutes remaining. He robbed Josh Kelly wide on the left before curling a brilliant shot past Graham from an awkward angle.
Four minutes later, and against all the odds, United were back in it when McCartan picked up a pass from Waide and crashed a low shot past Rory Brown from the edge of the box.
United were almost level on 84 minutes when McCartan's free kick was somehow flicked off the line by McClean, but he had he final sting in the tale when he slotted home a spot kick five minutes from time after Sean Graham was brought down by Chris Gallagher.
Ballymena United: Graham, Kane (Graham 17 mins), Redman, Addis, McElroy, McCartan, Waide, Hume, Ervin, Kelly, Wylie.
Unused subs: McKeown, Harpur, Knowles, Beattie, McCready, Farquhar.
Glentoran: Brown, Kane (Gallagher 79 mins), McCullough, Bigirimana, R Donnelly, McDaid, McClean, Clucas, Marron, Marshall, J Donnelly (Mitchell 81 mins).
Unused subs: Morris, McDonagh, O'Connor, Crowe, Plum.
Referee: Steven Gregg.
Man of the match: Shay McCartan.
Match rating: 7/10