Mount coach Paul Tietjens thought they had left it a little too late.
"We had to play a lot of rugby in that second half to come back," he said.
"Full credit to Rangataua. They really took it to us the way they attacked in the first half, they controlled the game and held onto the ball. They have a lot of smart rugby players and it was not until the end we were patient enough and got a great support try from our own 22.
"We have some good young players. Masiu Vainikolo from Western Heights High won a lot of ball for us at kick off and Manus Hanley came off the bench and played pretty well, plus we had the experience of Andy Devoy and Zar Lawrence in the midfield," he said.
"It is really special to win this. We had to beat Te Puke in the quarters and Whakatane Marist in the semis, with a lot of Fijians who know the game. I couldn't be more over the moon to win the old man's trophy."
Rangataua coach Walker was disappointed at the final outcome but pleased with his team's effort overall.
"In the end it was just one tackle missed and Mason got a ball away and that was pretty much the difference. We obviously wanted to win but we qualified for the (national) Middlesex Sevens (with Mount Maunganui) and that is another goal of ours to head down to Wellington and win that trophy back."
Other results: Women's Championship Exia Edwards Cup Waikite 12 Opotiki 5; Men's Plate Marist Saint Michael's 15 Greerton Marist 10; Men's Bowl Waimana 19 Opotiki 17; Tight Five Arataki 17 Judea 15; Men's Trophy winner Waikite; Men's Bowl winner Waimana; Colts winner Whakarewarewa.