Slick work from Rangiuru centre Elijah Nicholas, who had a ding-dong battle with former Auckland player Teddy Stanaway, put Rolleston over to give the home side a 10-5 advantage.
The lead grew to 15-5 when Rangiuru fullback Ngatai Kingi ghosted on to a Rolleston skip ball down the right to score, but breakdown infringing and an inability to combat the Mount pack allowed hooker Vyron Smith to score an unconverted try just before the break.
A 60m intercept try from Stanaway against the run of play changed the complexion in the Mount's favour after the break as their advantage up front, along with the measured distribution and option taking of replacement halfback Matt Golding, began to pay dividends.
Fullback Andy Devoy strolled over out wide soon after to widen the gap to 24-15, and when 18-year-old Miro Mitchell barged over it looked like things might get out of hand.
Rangiuru, though, stuck to their task, slipping outside the Mount's compressed left-hand defence a number of times.
Tries were there for the taking, but were not taken.
Carlos Basham dotted down in the shadow of the posts to close the gap to 31-22 with 10 to go, before Mount pivot Chad Jacobs went over untouched.
The Mount, set to be bolstered by the likes of Zar Lawrence, Chris Elvin and Johan Bardoul, looked organised in their second season under coach Rodney Voullaire and loom as true title contenders.