The Scotland striker had not featured since December due to a combination of injury and personal issues.
But he bounced back from his off-field problems to fire Celtic level as the Parkhead men recovered from losing a shock opener to thrash Austrian amateurs SC Pinkafeld 6-1.
Griffiths was joined in a youthful starting line-up by Ryan Christie, who was also making his comeback after suffering a horror facial injury in a clash with Aberdeen's Dom Ball during the William Hill Scottish Cup semi-finals, and captain for the day Scott Sinclair.
Celtic started in determined fashion but with the temperature soaring it was no surprise when they began to flag amid the Austrian heatwave. Pinkafeld took their chance to grab a surprise lead on the half-hour mark as 21-year-old goalkeeper Conor Hazard spilled a long-range Dominik Sperl free-kick straight at Michael Pahr, who slotted home from close range.
But the hosts' lead lasted just six minutes as Griffiths grabbed his first strike since November.
Anthony Ralston freed Calvin Miller down the right and his cut-back left Griffiths with a simple tap-in as he got back to doing what he does best.
Hazard did better with a Florian Prochazka strike as he tipped over while Griffiths almost grabbed a second before he and eight members of the starting line-up were replaced at the interval, firing against the base of the post following good play by Miller and Armstrong Oko-Flex.
Youngsters Karamoko Dembele and Mikey Johnston were introduced as Lennon made his sweeping half-time changes and it was the latter who fired the Parkhead side ahead within 30 seconds of the restart as the 20-year-old steered home with a composed finished.
Teenager Ewan Henderson grabbed the third after 53 minutes as his strike from the edge of the box squirmed through Pinkafeld keeper David Kayer's hands.
And with Pinkafeld tiring, Englishman Oko-Flex got himself in on the action with six minutes left after producing a cute dink to finish off from Johnston's sublime through ball.
Jack Aitchison and Johnston then rounded off the scoring with another two goals late on.
Celtic return to action on Saturday against Austrian third division outfit Wiener SC.