Tommy Wright's side were without five first-team regulars as they kicked off their Betfred Cup campaign against Stewart Petrie's part-timers.
And it was the League One Gable Endies who finished the strongest as Masson fired home the winner midway through the second half to seal the biggest shock of the tournament so far.
Saints had not kicked a competitive ball this summer before pitching up in Angus, yet Wright was already faced with a lengthy casualty list.
Matty Kennedy, Drey Wright, Liam Gordon, Chris Kane and David Wotherspoon all missed the Group B opener, with Zander Clark, Liam Craig and Steven Anderson only fit enough for the bench.
New signings Elliot Parish and Wallace Duffy came in for their debuts but the ring-rust was obvious as St Johnstone stuttered into life.
Lewis Milne almost took advantage of that sluggish start but his early set-piece and headed attempt flew wide.
The Perth team eventually found their feet and should have grabbed a first-half lead when Ross Callachan swung a free-kick in from wide but Callum Hendry failed to get a killer touch.
Michael O'Halloran tried to force the issue after the break but could only fire at Allan Fleming.
But Montrose were still very much in the fight.
It took Parish to dive full length to his right to stop Paul Watson smashing Montrose ahead on the hour mark – but the hosts did grab the winner with 18 minutes left.
Duffy was bundled out the way by Blair Lyons in the far corner and the substitute's cut-back was swept into the top corner by Masson's sweet strike.
Perth youngster Kyle McLean took to dubious means as he grasped desperately for a Saints lifeline.
But he was not fooling referee Barry Cook as he tumbled at Sean Dillon's feet inside the box and was rightly booked.
David McMillan came close to levelling in stoppage time but was foiled by keeper Allan Fleming.