He scored his second goal of the tournament to give Nigeria the lead after 19 minutes.
Peter Etebo swung over a cross to the far post where Kenneth Omeruo pushed the ball back across goal. Ighalo stabbed in via a deflection that foxed Cameroon goalkeeper Andre Onana.
But two goals in three minutes from Stephane Bahoken and Clinton Njie handed Cameroon the advantage before the pause.
Clarence Seedorf’s men started the second-half with purpose aiming to extend their advantage but they were undone twice in three minutes.
Ighalo thrashed in his second of the game - and his third of the competition - from Ahmed Musa’s neat chested lay-off. Oghalo then turned provider with a sumptuously weighted pass allowing Alex Iwobi to sweep the ball past the Onana.
Cameroon were befuddled and would have conceded a fourth but for Onana’s reflexes. He clawed away a goal-bound header from Igahalo and parried away another effort from the Shanghai Shenhua forward.
Following his brace, the 30-year-old is joint top scorer at the tournament with Senegal’s Sadio Mané and he will be able to increase his tally in the quarter-final against Egypt or South Africa.
While he and Nigeria tilt at more glory, Seedorf’s future will come under further scrutiny. The 43-year-old Dutchman has failed to convince since taking over from Hugo Broos who led Cameroon to the 2017 Cup of Nations title.