Linfield's title charge hit the buffers in a night of drama at Mourneview Park.
With only five games in an intriguing Danske Bank Premiership campaign remaining, their advantage at the top of the table has now been reduced to an alarming seven points.
The pressure is now on David Healy's title holders, with Big Two rivals Glentoran - whom they face next weekend - now in hot pursuit. The cheers from east Belfast were almost audible in Lurgan when referee Ian McNabb gave the final blast on his whistle after this thriller.
Make no mistake about it, Gary Hamilton's Glenavon were well worth their victory.
Veteran defender Colin Coated headed them into the lead with his 74th goal in 600 Irish League games before Shayne Lavery levelled.
But finishes from Glenavon's deadly duo Matthew Fitzpatrick and Danny Purkis put them out of sight, even though substitute Christy Manzinga reduced the deficit with 18 minutes left.
"It was a poor performance and a poor result - it was a day when everything went wrong," said Healy.
"We started the game poorly by giving a soft first goal and that set the theme.
"The second goal was even worse. From that point, we were chasing the game. Glenavon are the type of team that will exploit you if they sense you are a little bit soft - and they did that.
"Our level of performance was nowhere near what is expected at this club. It's something we can't brush over. The players are honest, they know they fell short. They are disappointed with themselves. We didn't deserve anything from the game.
"Sometimes you can carry one or two players, but we had so many that just weren't at it - we carried too many, that's for sure.
"We are now going into the last five games with an advantage, albeit I was hoping we would be three points better off. But if you had offered me a seven-point advantage at the start of the season, I certainly would have taken it.
"There is a lot of hard work to be done, that's for sure, but my only focus now is the Irish Cup tie in midweek with Annagh United."
Lavery had a header deflected over the crossbar by James Singleton early on before he connected with a Kirk Millar cross, but that one flew over the top.
It was Glenavon who forged the breakthrough on nine minutes when Peter Campbell whipped in a free kick from the right which was flicked home by Coates.
The home side managed to hold the advantage for only four minutes. After missing those early chances, this time Lavery was unforgiving. He reacted first to meet a Matty Clarke corner kick and his header fizzed past Craig Hyland.
Linfield almost struck again seconds later, with Millar heading Clarke's cross into the six-yard box, but Jordan Stewart's header was saved by Hyland.
Glenavon regained the lead in bizarre circumstances just before the half-hour. Clarke's clearance ricocheted off Matthew Snoddy and spun into the path of Fitzpatrick, who stabbed past a startled Johns.
Healy's team had a half chance to level just before the break but, after cutting past Sean Ward, Joel Cooper could only fire his right-foot shot into the side netting.
Glenavon really should have been out of sight on 59 minutes. The lively Campbell produced a great solo dash on the left and, when his cross fell to Fitzpatick, he took a touch before blasting his shot wide.
They had better luck two minutes later. This time Purkis lost his marker when James Singleton produced a delicious delivery and his flicked header gave Johns no chance.
Healy responded by calling for the calvary in the form of Manzinga and Andy Waterworth - and it almost paid an immediate dividend when the latter met a Stevie Fallon cross only to see Hyland flip his header over the top.
But they were back in it with 18 minutes left when Ward hauled down Lavery and Manzinga curled in a stunning free kick from wide on the left that totally deceived Hyland.
Glenavon boss Gary Hamilton insisted his boys got what they deserved.
"I thought we were good from start to finish, especially the front two, Fitzpatrick and Purkis - they led the line brilliantly," he said.
"They caused Linfield problems all day long. I also thought Peter Campbell was exceptional, he was the best player on the pitch.
"Unfortunately, the performance still wasn't good enough to get us into the top six. We knew our fate even before kick-off, but the boys still put in a massive shift. Our aim is to stay in seventh place to get us into the European Play-Offs."