Mount had further chances but were unable to double the score.
Otumoetai's Cory Bruin made a fine run through the Mount defence and looked to be tripped inside the circle, but the umpire closest to the action waved away the protests.
The game meandered along until two goals in two minutes livened everyone up.
First Jono Simmonds placed his shot accurately from a penalty corner to even the game up 1-1 before a freakishly good finish by an off-balance Dan Rae restored the Mount's one goal advantage at halftime.
Both teams lifted the intensity and pace of their play after the restart.
Otumoetai had an early chance but after quick transition from the back, Mount's Joseph Lyons slipped his marker and delivered a pinpoint ball for Logan Thompson to score.
Down 3-1 the defending champions fought back with a goal to Hugh Gilmer, after a clean strike from a penalty corner, to close to 3-2. There was no late rally for Otumoetai though and Mount's NZ Under-18 player Hamish McGeorge flicked his shot just wide to finish the action.
CMS Mount 3 (John Preece, Dan Rae, Logan Thompson) Bayleys Otumoetai 2 (Jono Simmonds, Hugh Gilmer)