Lookman scored both goals, the first created by Victor Osimhen, and the second set up by Calvin Bassey after a free kick in the last minute.
But the fans' hero was Osimhen, who produced a tireless performance of running, attacking and harrying the Cameroon defenders. Osimhen also helped in defense on the few occasions it was needed.
"Osimhen! Osimhen! Osimhen!" rang around Felix Houphouet Boigny Stadium after the Napoli star won the ball off defender Oumar Gonzalez after a Cameroon throw-in and set up Lookman in the 36th. The ball squirmed inside the right post despite Gonzalez's attempt to keep it out.
At one stage in the second half, after Osimhen battled lone defender Gonzalez for the ball near the sideline and it went out of play, he lifted his arms to encourage the fans and they responded by chanting his name again.
Nigeria goalkeeper Stanley Nwabali went off on a stretcher in the 80th after a reckless challenge from Georges-Kevin Nkoudo, who was booked. Nwabali, who covered his face, was replaced by Francis Uzoho.
Cameroon hero Vincent Aboubakar went on for the last quarter-hour but it was Lookman, instead, who sealed the result at the other end.
Aboubakar, who scored the winning goal in the 2017 final and was the top-scorer in the last edition with eight goals, missed Cameroon's opening three games with a thigh injury.
Nigeria will next play Angola.
Additional reporting from AP