Guy Bates and Mark Farren both bagged a double as Glenavon put six past Ballymena at the Showgrounds.
Bates netted the opener with a deflected finish and he added the second before goals from William Murphy and Farren made it 4-0 at the break.
Neil Lowry and Gary Thompson scored as United threatened a comeback but Glenavon restored their four-goal lead through Farren and Andy McGrory.
Johnny Taylor headed in a late consolation for the Sky Blues.
It was a dream first half for the Lurgan visitors, starting on seven minutes when a shot from Bates deflected off United's Taylor and looped into the net.
Bates met Gareth McKeown's long throw-in at the near-post to nod in the second goal after 27 minutes.
It was quickly followed by the third and a goal on his debut for Murphy, who supplied a composed finish after the hosts failed to clear a free-kick.
Farren made it 4-0 after just 34 minutes, his turn and shot finding the bottom corner.
Ballymena pulled one back seconds after the restart with Lowry drilling in and Thompson gave the Showgrounds faithful hope when he volleyed home on 65 minutes.
The next goal was vital and it went to Glenavon, Farren's confident finish followed by number six from McGrory.
Taylor's late goal could not disguise a miserable start to the campaign for Glenn Ferguson's men, while Glenavon will aim to build on an impressive display.
"We did so well in the first half but we were hanging on a bit in the second half," said Lurgan Blues boss Gary Hamilton.
"We are determined to make progress and you have to be happy with three points on the first day of the season."