It was former Derry City striker Junior and the reliable Jay Donnelly who hit the goals to earn the East Belfast side the three points.
It was another night of misery for the Sky Blues, who had to play the last 18 minutes with only 10 men after Sean Graham was dismissed.
United’s Johnny McMurray had the first sniff of goal after only four minutes when he met a TJ Murray cross – only to see his header deflect over the crossbar, off the head of Paddy McClean.
Glentoran showed how it should be done three minutes later. Shay McCartan’s corner-kick caused chaos in the United defence, with James Singleton and then McClean getting vital touches, before Junior displayed his finishing instincts with a neat strike.
Donnelly really should have had them two up just after the half-hour. After picking up a Fuad Sule pass, he did all the hard work but, just as he was about to shoot, Scott Whiteside dived in with a goal-saving tackle.
Donnelly missed another good opportunity seconds later when McCartan’s corner-kick found the striker at the back post, but he could only head wide.
The Glens carved out another good chance just before the interval when goalkeeper Aaron McCarey’s long clearance caught the United defence napping.
Donnelly cleverly flicked the ball on for defender Marcus Kane and, with only Sean O’Neill to beat, he could only lob his shot over the top.
It was as you were after the restart with Glentoran asking all the questions.
McCartan failed to maximise two long-range free-kick opportunities and O’Neill was then forced into a smart save on 55 minutes, getting down low at his post to keep out an effort from Daire O’Connor, following a great delivery from Donnelly.
Then McCartan burst through the middle before letting fly, to which O’Neill was equal.
And the chances just kept on coming - Donnelly was presented with another opportunity on 66 minutes courtesy of Nelson’s miskick but again he was off-target with his half-hit shot.
United’s night was compounded when Graham was dismissed 18 minutes from the end after picking up his second yellow card.
Donnelly at last sealed the points on 94 minutes. Substitute Niall McGinn crossed from the right and the striker scooped home, much to the delight of the big travelling support.
BALLYMENA UNITED: O’Neill, Nelson, Whiteside, Graham, Waide, McCullough, McGuigan (Tennant, 77), Taylor, Murray (Rocks, 46), McMurray (Gibson, 77), Place.
Subs (not used): Johnston, Crawford, McMullan.
GLENTORAN: McCarey, Kane (Russell, 46), McCullough, McCartan (McGinn, 88), Donnelly, O’Connor, Singleton, Sule, Palmer (Kelly, 80), McClean, Junior.
Subs (not used): Webber, Boyd, Wightman, Walsh.
Referee: Shane Andrews.