Johnson then followed up that eagle with three consecutive birdies from the 14th to seal a second win at Kapalua after his first in 2013.
The Sentry Tournament of Champions had reached that stage where all that was left to decide was the margin by which Johnson would win.
Dustin Johnson has a long game and a short memory. Similarly, Jordan Spieth has fallen out of striking distance when he was only able to manage a three under round of 70. "I hit it a little thin", Dustin Johnson said. "I kind of just kept the pedal down".
Harman also closed with a birdie-four to card a 69 and ensure a place in Sunday's final pairing with Johnson, while Rahm will head out in the penultimate group after his grandstand finish to his 66.
Well, the PGA Tour's Twitter account on Monday posted a video of a number of near-hits that other players - from the legendary Nick Taylor to two-time major champion Zach Johnson - have enjoyed over the past several years.
Spain's Jon Rahm took second spot, shooting a four-under 69 to finish on 16 under, while Brain Harman came in third one shot further back after a 72 that included three bogeys on the front nine.
The way Johnson was playing, everyone was playing for second. "I didn't want it to have to come down to the last couple holes", he said. "More for myself and confidence, after what happened in Shanghai, I wanted to prove to myself that I still. that ain't going to happen again".
The extraordinary drive and resulting eagle put an exclamation point on what had already been a dominant performance and he put the icing on the cake with three more birdies on his way to 24-under for the week and a 17th career PGA Tour title.
When it comes to smashing a golf ball really hard, England's Joe Miller is one of the world's best and his performance at the World Long Drive Championship in Thackerville, Oklahoma in 2016 blew us away. "I knew once it got on the green that it was going to be pretty good".
"But everything's going in the right direction".