They defeated Wellington 3.5-1.5 but were looking like underdogs for the championship final with Bay of Plenty, who demolished Otago 5-0.
But McDonald said in the matchup that counted, the team caught fire.
"The whole way through it was just red for Manawatu-Wanganui we were just up the whole way.
"To beat them that convincingly in the final was awesome, we didn't want to leave it up to chance like last year."
The historic nature of the victory was also recognised by their peers as the other representative teams lined up and gave rousing applause to the emotional winners as they walked back to the clubhouse.
"Everyone clapping us up the hill, the whole team environment is so awesome," said McDonald.
"There is a lot of people who have supported all of the guys in this team for many years. This win is for them.
"We know that there has been many great players come through the province over the years and never won this title.
McDonald will now practice for the rest of December before heading to Australia for the Amateur Championships next month, taking a lot of confidence from his form.
"I have worked really hard on my game and to win the [player] award in this company confirms that I am on the right track. I couldn't be happier."
After beating Canterbury and drawing with Otago in Rounds 3-4, Manawatu-Wanganui's date with destiny nearly got derailed on a nerve-racking Friday when they lost 3-2 to Northland then halved the matchup with Tasman.
"If [Tasman player] Blair Riordan's ball had rolled one more revolution in his match against Josh [Munn] then we were gone," said McDonald.
"But we made it through and I guess as they say the rest is history."
Munn, 22, missed a birdie chance from 20ft to win the Tasman matchup, then had to sweat through Riordan's putt attempt from 10ft which also came up just short.
"We thought we'd blown it but we are stoked that a half was enough," Munn said.
He made no mistake in the final against the previously undefeated Bay of Plenty No1 Peter Lee, with Munn's 5-4 win setting the tone for his team.
"I chipped in on the first hole and it gave me the boost that I needed.
"From there I played really well and it was the best feeling to walk past the board at the turn and see it all covered in red numbers [for Manawatu-Wanganui].
"We haven't won this in almost 50 years," Munn enthused.
"I can't believe that, that is a total joke and I am just pleased we ended that drought. This is a great team and we showed that today."
Perhaps the win meant the most to manager and selector Russell McDonald, who has been with the squad for a decade.
"These boys have worked so hard for this and the way they won it in the final was just incredible," he said.
"I have worked with all of these boys for many years and to see them fulfil their potential on this stage is so rewarding.
"They are such a great team, great mates. They do everything for each other and they never give up. That really showed this week."
Fifteen representative squads from around the country contest the Toro Interprovincial during a week of games, playing twice a day.