The South Australian clocked 1:03:02 on the 52.5km course, one minute and 21 seconds too good for Tom Dumoulin, while only about five-tenths of a second split the Dutchman and bronze medallist Belgian Victor Campenaerts.
A multi-world champion on the track, a tarmac-forged rainbow eluded Dennis, who has suffered his share of bad luck in past Olympic and world title tilts.
"I've been chasing this since I was a junior," he said. "I've never won it in any age group so to win my first one in the seniors is really special. I can't really explain it. It's an amazing feeling."
While quietly confident of his numbers, Dennis still felt the nerves before the climb.
"I was being coached from the car by Brad McGee and he gave me confidence at the top of the climb once Dumoulin came through that I was a minute up and it just kept me calm.
"I was considering a bit of a victory salute but I wanted to make sure, you're never sure until Tom does cross the line."
And this one's for the power/numbers geeks
"I knew the power we rode in the team time trial and we lost 19 seconds. And I held high power up the hill than I did the team. And I knew he rode the front and he was pushing pretty hard in the team time trial so I had a fairly good idea what power to hold.
"To hold him I think I was 35 seconds at the bottom and when I got to the top I was fairly confident that I'd at least done the same time as him."
In his post-race winner's interview, Dennis' first thoughts were with his wife and former cyclist, Melissa Dennis.
"There's a lot of people I have to thank for this and the first one's my wife. She's back at home. I'm sure she's pretty happy at the moment."
He was also uninterested in comparisons to Michael Rogers, who won the ITT title two more times after winning his first in 2003.
"I'm not too stressed about beating a record. I'm just going to enjoy it at the moment and enjoy the next year in rainbows and try again next year."
Asked if the 2018 La Vuelta was the right preparation for securing his title, Dennis quipped "I won so I guess it is!
2018 Tour de France runner-up and visibly race-trim Dumoulin placed the credit in the correct place as he looks ahead to Sunday's road race.
“I arrived on the start line with the goal of being the world champion again but Rohan [Dennis] was outstanding, that’s my story of the day.
"It’s been a physical and mental struggle for an hour. I wouldn’t be second if I didn’t fight my way until the finish. Sometimes you win, sometimes you lose.
"I had a bad time trial in Richmond three years ago and I had my best road race after that, so I’m going in it with an open mind on Sunday.”