Frooms opened the scoring with a clinical finish to a good team move by meeting a Sam Patterson right-wing cross as it streaked across the face of the goal, five metres out.
Ten minutes later, the ball came in from the right again, a goalmouth scramble ensued and Land got the decisive touch.
In the 70th minute, left-flank midfielder Travis White, having come on for Brandon Josling 10 minutes earlier, put Thistle 3-0 up.
Darting in from wide, White ran on to a through ball from Frooms, picked his spot and put the ball away.
Marist scored a consolation goal from the penalty spot 10 minutes from time, for handball given against leftback Daniel Venema.
Thistle coach Garrett Blair said the boggy surface and rain, which fell for much of the second half, meant the Jags had to change their tactics.
All but a few tactics fall apart when the ball gets stuck in the mud.
Nevertheless, Thistle generally had the better of play, except for a 20-minute spell in the second half when Marist mounted a strong effort to close the gap.
Thistle were composed at the back, with centreback Ander Batarrita outstanding. He was well supported by fellow central defender Ema Martos, and fullbacks Emerson Araya and Venema.
Goalkeeper Mark Baple did not have a lot to do but made one excellent save. Marist, 3-0 down, counter-attacked and their striker hit a shot towards the bottom right corner of the goal. Baple dived to his left and deflected it wide, where Venema got on to it and cleared the danger.
While Blair described the game as “scrappy”, the players did not lack commitment. The slippery conditions meant sliding tackles had to be well timed and executed not to be judged reckless, and the referee showed the yellow card to “about three” players in a keenly competitive Marist side.
Thistle skipper Land and Ashley McMillan had a running battle with their Marist counterparts in central midfield, while Patterson on the right and Josling on the left tried to make the most of the better going out wide.
Nineteen-year-old Tomek Frooms had another good outing in a deep-lying striker's role, Blair said.
“They didn't know what to do with him.”
Up front, Davie Ure got into scoring positions and had his chances but, this time, was unable to turn them into goals.
Frooms and Ure both seem to thrive in the deep-lying striker's role. Both are in good form. How to use them together and make the most of their gifts is a question bursting with possibilities.