North, one of the stars of the Lions' series victory over Australia in the summer, collected a kick from Karl Dickson and skinned Quins winger Charlie Walker to score.
From there Saints pulled away with Dylan Hartley adding a late poacher's try to triumph 23-9 and go top of the Premiership table.
"We needed it (George North's try). He was one on one and it was fantastic," said Mallinder.
"It was against young (Charlie) Walker, who is a talented player, but George, one on one, you've got to make sure you make a good tackle.
"To see him go in the corner was fantastic. It's pleasing (to have done the double over Harlequins) because there was all that chat last year about us not beating teams and it hurt."