Despite jumping into a two-goal lead, Rangers looked unconvincing in a patchy first half, and St Mirren were buoyed by a goal before the interval.
After the break, though, 27-year-old Penrice typified a much sharper display from McInnes’ side, driving a constant threat down the left flank.
That was evident on 52 minutes, when the full-back burst up the left on the counter and slid a precise first-time pass to Djeidi Gassama.
Two minutes later, Penrice scored himself, crashing in a loose ball from close range to restore a three-goal advantage.
Former Rangers striker Steven Thompson told BBC Radio Scotland’s Sportsound that Penrice not only found the net but produced an outstanding debut performance—an added bonus for the club.
He noted a clear shift in the atmosphere inside the stadium and said the result would lift everyone at Rangers.
Plenty of positives for Derek McInnes.