Proctor, whose sole appearance for New Zealand came a year ago, was making his Premiership debut and soon endeared himself to the Franklin's Garden crowd with an early brace in the derby.
Saints were without international trio Courtney Lawes, Dan Biggar and Owen Franks because of injury, yet they were 24-10 up at the interval, Proctor crossing twice and then producing the break that led to Api Ratuniyarawa going over.
There was no way back for Leicester, despite Guy Thompson's first-half score, as Tom Collins and Fraser Dingwall added further tries for Northampton after the interval.
Exeter Chiefs moved up to second, four points adrift of Saints, with a comfortable 38-3 success over Wasps.
Having lost to Bristol Bears last time out in the Premiership, Exeter bounced back in style in a five-try display, with England international Luke Cowan-Dickie among those to go over.
Fourteen-man Sale Sharks was beaten 20-13 by Worcester Warriors as the visitors were left to rue Jean-Luc du Preez's red card for a shoulder-led tackle on the stroke of half-time.
Sale led at the break, Faf de Klerk and Chris Ashton having gone over, but Perry Humphreys and Ashley Beck both crossed in the space of two minutes after the Sharks went down to 14 men, with the boot of Duncan Weir, who kicked two conversions and a pair of penalties, proving pivotal.