Goals from Jordan Henderson, his first in seven years, Mohamed Salah, Ante Rebic, Brahim Diaz and an own-goal from Fikayo Tomori saw Anfield witness another European classic.
Steven Gerrard was still the captain the last time Henderson found the net in a Champions League game at Anfield and it was fitting that he produced a goal the Liverpool legend would be proud of.
He smashed the ball from 20-yards out into the bottom left hand corner past AC Milan goalkeeper Mike Maignan to seal a fine comeback for Liverpool.
The Reds made the perfect start early on thanks to a Tomori own goal after he diverted the ball into his own net from a Trent Alexander-Arnold cross.
The score line should have made more comfortable reading and it would have been had Salah converted from the spot, something he had done on 17 previous occasions.
Eventually Liverpool tired and AC Milan settled and against the run of play a quickfire double from Rebic and Diaz saw the visitors go ahead at the break.
Liverpool and Salah did not let that affect them though and the Egyptian redeemed himself by equalising early on in the second half.
The comeback was then complete 20 minutes from time as Henderson sealed a vital victory for Liverpool with a thunderous strike.
Although the finale should have been far more comfortable, manager Jurgen Klopp will be delighted with the character his side showed as Liverpool begin their campaign with maximum points.