The 49-year-old has long been credited with playing a huge part at Clarehaven Stables, first for John Gosden and more recently in his partnership with his son, Thady.
An impressive winner of the Rockfel Stakes just two weeks ago, Commissioning was sent off the 8/13 favourite to remain unbeaten and cement her 1000 Guineas claims.
Having travelled sweetly for the first half of the contest, she came under pressure two furlongs out as Karl Burke's Novakai made a bold bid from the front.
But Commissioning relished the step up in trip and her most impressive furlong was her last, as she ultimately ran out a clear-cut one-length winner. Novakai's stablemate Bright Diamond was third.
Frankie Dettori, serving the final day of a suspension, was on course to witness his friend gain such a notable success and told ITV Racing: "He's been a massive part of Clarehaven for over 20 years.
"He's 49 years old and has never won a Group One so I could not miss this, to watch one of my best friends win a Group One.
"Rab is a key part of our team, most of my success is down to Rab as he's prepared most of my best horses all my life. He's a personal best friend and I'm delighted for him - finally he's done it!"
Havlin said: "We got there in the end. Frankie needs to get banned more often!
"She gave me a worrying moment in the dip as she got a bit unbalanced, but when she hit the rising ground she took off. She's still a long way from being the finished article mentally, but she can get herself out of trouble. I'm glad to get the job done, that was a relief.
"She's got the tactical speed for a Guineas. I rode her mother and she would have stayed a mile and a half, but she only had two runs.
"Hopefully the next Group One doesn't take another 30 years!"
The veteran Scot admitted he wondered whether his day in the Group One sun would come, particularly after being mowed down late aboard Commissioning's stablemate Laurel in last weekend's Sun Chariot over the same course and distance.
"I'm elated - I don't think it will sink in until I get home," said Havlin, who steered great names like Enable and Stradivarius to victory early in their careers.
"I just tried to fill my morning with different things and didn't really think about it, but everyone was wishing me good luck.
"I had a plan in my head and changed it about three times before I went out! I just said I'd wait to see what happened when the stalls opened as I didn't want to be a prisoner to any plan.
"Obviously you feel a little bit of pressure, but once the stalls open everything goes out of your head and you do what you've been doing for the last 30 years.
"I was thinking last week 'would it happen', as everything went like clockwork on Laurel and somebody came and nabbed me - I couldn't believe it. I don't think I could have done anything different in that race, it was just one of those things.
"It's hard when the whole race goes to plan and you still get beat, but anyway, we've got one now."
John Gosden said: "There was a strong pace and a big old crosswind, which got her a bit unbalanced.
"Coming into the dip she changed legs, but when she met the riding ground she took off well and I think it took to the Cambridge Road to pull her up, so that was great.
"But for Rab Havlin, nobody deserves it more. It's a fantastic achievement, a bit like Sir Mark (Prescott) with his Arc last weekend."
Thady Gosden echoed his father's sentiments, saying: "Everyone loves Rab - you couldn't find a better man and nobody deserves it more.
"The filly has got plenty of speed and is by Kingman, but the mother is by Galileo and she stayed well.
"We'll see how she is in the spring and how she's training, but she's obviously a filly you'd look towards a race like that (1000 Guineas) for."