A winning goal from Kevin Braniff and defeat for Cliftonville saw Glenavon secure third spot and a place in the Europa League.
The Lurgan Blues claimed the vital win thanks to a second half strike from Braniff to see off Glentoran and ensure that they could not be caught with just one game to go.
It was a fairly mundane start with little action as both teams took time to settle on the hard surface.
Curtis Allen tried his luck from a 20 yard free kick but Jonny Tuffey gathered easily. On 11 minutes Glenavon had their first effort on goal as Ciaran Martyn hooked a speculative shot over the bar.
The home side went close on 19 minutes when a corner fell to Jay Magee and his swweetly struck volley flew inches over the Glenavon bar.
At the other end Glenavon were having to shoot from distance but efforts from Eoin Bradley and Kevin Braniff brought no reward.
A scrappy game came to live on 30 minutes with Braniff playing a brilliant ball out wide to Andy Hall, he surged forward and tried to pick out Bradley but Birney nipped in to clear and prevent Glenavon from scoring.
The Lurgan Blues were finally beginning to look more of a threat going forward and from a free kick Braniff beat the wall but his effort flew over the bar.
There was another chance for Glenavon to edge ahead just before the break when a poor clearance fell to Martyn, but his way to goal was blocked, Bradley took over but again it all came to nothing.
There was a lively start to the second half with the Glens getting in behind and David Scullion heading wide inside two minutes.
Then Bradley and Cooper found space on the left but with Martyn free in the centre they could not get the pass in.
On 54 minutes Glenavon missed a sitter, again neat approach play by Bradley and Hall saw the ball played in to Braniff almost on the goalline but he scuffed his shot over the bar.
Braniff made up for that miss on 65 minutes with a sublime finish to finally make the breakthrough. It came after a quick throw-in from Cooper caught the Glens defence sleeping and Braniff made no mistake with a perfect finish.
It was almost 2-0 on 77 minutes after Hall burst into the box but Morris brilliantly save his stinging shot.
With nine minutes to go Glenavon brought on Daniel Kearns and Fabio for Braniff and Cooper with player manager Hamilton himself coming on for Hall a few minutes later.
Despite a nervous last few minutes Glenavon saw the game out for another famous win.
Glenavon: Tuffey, Marshall, Dillon, Lindsay, Patton, Cooper, Kilmartin, Martyn, Hall, Braniff, Bradley.
Subs: Hamilton, Kearns, Sykes, Fabio, McGrory.