The emphatic win came 16 years to the day after Boston’s last NBA Finals win – over the Lakers – in 2008.
“Oh my God. It’s a surreal feeling. We did it,” Tatum said. “We did it – oh my God, we did it!”
“First of all, God is the greatest, not ‘cause we won, but to put me in positions to maximise my God-given ability to surround me with these guys and with my family.
“This is an incredible feeling. I’m lost for words. I’m sorry.”
Jaylen Brown, who was named the finals MVP, gave Tatum scoring support with 21 points, while Jrue Holiday added 15 and Derrick White 14.
The defeat marked a miserable end to the finals for Dallas duo Luka Doncic and Kyrie Irving, who had led the Western Conference fifth seeds on an improbable giant-killing journey to the NBA Finals.
Doncic, sensational during the Mavs’ game four rout of the Celtics, finished with 28 points.
But Irving scored a meagre 15 points while being taunted with chants of “Kyrie sucks!” by Celtics fans still rankled by his departure from the team in 2019.
Top-seeded Boston meanwhile reasserted their superiority in convincing fashion after their game four blowout, leading by double digits from late in the second quarter until the final buzzer.
The Celtics laid the foundations for victory with a controlled first-half performance that saw them open up a 67-46 lead at the break.
Payton Pritchard put the seal on a superb first two quarters for Boston, launching a mammoth buzzer-beating three-pointer from near halfway that gave the Celtics a 21-point advantage.
Boston had been swiftly into their groove early on, building a 9-2 advantage in the first quarter with their swarming defence never allowing Dallas to build any kind of rhythm.
After Boston took a 13-5 advantage the Mavs rallied to narrow the gap to two points at 17-15, but Boston’s defence reclaimed the initiative soon afterwards.
Two quick steals from Tatum and Sam Hauser allowed the Celtics to pull away to 28-18 at the end of the first quarter.
Boston’s defence gave no respite to Irving and Doncic, restricting Dallas’s stars to just 14 points between them in the first quarter.
By contrast, Tatum and Brown threatened to overrun the Mavs defence.
There was no let-up from Boston after the restart, with Holiday’s cutting lay-up putting Boston 26-points clear at 78-52 early in the third quarter.
There were flickers of a Dallas recovery in the latter stages of the third quarter, but Boston remained in control to lead by 19 heading into the final period.
With the TD Garden roaring them on, there was never any chance the Celtics would be denied and Boston continued to find mismatches to score freely and retain a comfortable double-digit advantage to close out the win.