But while the home side did manage one try before halftime for 15-5, they got no closer thanks to Collegiate's solid defence in the pack, stopping the pick-and-go, and with around 15 minutes left, the home team had punched themselves out.
Collegiate's backline had always looked dangerous if they could just get the ball, and with Collegiate now having better of the exchanges up front, they had chances to spread wide.
Mason dived over for his double, and then added another penalty for 25-5, and Wellington were gone.
Standout No 8 Semi Vodosese scored in the shadow of fulltime, with Mason converting again for a 22-point haul.
The Steve Simpson-coached side will now make their second major final in two years on Wednesday, in what will be a rematch from last year's opening game with Nelson College, who impressed in their 24-14 win over defending champions Christs College today.
Tournament history had favoured Wellington, with the school having won or co-won the Quadrangular some 37 times since 1925, which included a record unbeaten streak of 12 times from 2003 to 2014.
However, compared to a Collegiate team on the rise, the 2019 edition of the home side has struggled in their games this season – able to move forward through a few phases but then either failing to convert pressure or losing possession.
Nelson College do not have that problem – being currently ranked in the top 10 of the best 15 school teams in the country by the NZ Herald's weekly 'First XV Rugby Report'.
They were the 2017 Quadrangular Tournament winners, but brought a young side to Whanganui last year when Collegiate upset them 19-13 with a late comeback in the opening game – Vodosese scoring the match-winning try – before Nelson regrouped to thump Wellington College 47-0 in the minor final.
Collegiate, who lost the 2018 major final to the Reuben Thorne-coached Christs College 33-7, will now face most of those same Nelson players from last year, except they are now older, stronger, and hungrier.
The underdogs can expect a much tougher test in the lineout and the phase work.
Tomorrow is the day off with the tournament teams scheduled to visit Government House as part of the festivities, and kickoff on Wednesday for the major final is 1.30pm.