The no.7 kept his cool from the penalty spot after being hauled over by Max Aarons to earn three more precious points.
It means Ralf Rangnick has now secured maximum points from his two Premier League games in interim charge of the Reds.
After a tight first half, when Alex Telles hit the bar with a free-kick but the Canaries comfortably kept United at bay for long periods, David De Gea made a stunning stop to palm a Teemu Pukki drive over the top.
Following Ronaldo's penalty, De Gea made another great save to flip Ozan Kabak's header away from goal as the home side looked to get back on level terms.