Glentoran hit the front early thanks to a mishap in the Coleraine defence.
Danny Amos delivered a superb cross from the left that Coleraine defender Charles Dunne could only direct into the back of his own net after only three minutes.
Ten minutes later it was almost two. Jordan Jenkins raced onto a long ball over the Coleraine defence, and he tried to lob the retreating Rory Brown in goal from 30-yards out, but the ball dropped wide.
Despite absorbing lots of pressure, Coleraine could have been back on level terms on 21 minutes.
Fuad Sule chopped down Jamie Glackin inside the area with referee Shane McGonigle pointing to the spot. Up stepped Matthew Shevlin but his effort was saved superbly by Daniel Gyollai.
Moments later David Fisher flashed an effort from the left, but it crashed off Brown’s right-hand upright and away to safety.
Three minutes after the break Coleraine went close as Shevlin rolled the ball across goal but first Jamie Glackin and then Rhyss Campbell failed to get a clean connection on the ball.
Despite Coleraine’s pressure, it was Glentoran who struck a second on 57 minutes.
Dylan Connolly, who had just arrived on the pitch moments before, sent a low drive from outside the area that whistled into the bottom left-hand corner of the goal, giving Brown no chance.
Coleraine grabbed a consolation through Declan McManus’ penalty in injury-time after Shevlin was brought down in the box but it was not enough.