Canning twice hit the target late on — including one from a sideline cut — to clinch the 1-16 to 2-11 win for the three-time All-Ireland champions as 1-4 from Conor Cooney was not enough to rescue the 2012 champions.
Damien Hayes set the tone for the Portumna victory when he goaled after just 90 seconds and helped his side into a 1-8 to 0-3 lead.
However, goals from Conor Cooney and Gerald Murray brought the holders back into contention by the break as they only trailed by 1-10 to 2-4
Cooney continued to shine in the second half and, aided by county colleague David Burke, he hauled the champions back into contention, however Canning twice split the posts for Portumna to leave the champions with an uphill task to reach the quarter-finals.
Galway skipper Fergal Moore made his first return to action for Turloughmore since being knocked out in the league semi-final, but he was unable to help them to the win as they ended level with Gort at 1-16 each.
It was Conor O’Shaughnessy who saved Turloughmore’s blushes with a last-second equalising point after Gort looked to have clinched the win thanks to Keith Killilea’s late goal.
Also in Group B action, Maurice Callanan hit 0-13 for Ardrahan as they picked up a 0-23 to 0-19 win over Mullagh.
Johnny Coen and Johnny Maher both hit late points for last year’s runners-up Loughrea as they took a good first-day win over Kinvara by 0-17 to 0-13.
Meanwhile in other Group C action, Castlegar edged an epic encounter against Craughwell 1-17 to 1-14. Dean Higgins hit the net in the closing stages to clinch the win for Castlegar .
Eanna Ryan top-scored for last year’s intermediate champions Killimordaly, but they needed a late Keith Kennedy free to seal a share of the spoils against Clarinbridge, while Padraig Pearses also got their campaign off to a flyer with a 0-16 to 0-13 win over Kiltormer.