However, he finally achieved his goal on what proved to be an emotional day in Massachusetts.
Speaking to Sky Sports, he said. "Unbelievably emotional. Forty years caddying; I knew somebody would get that giant monkey off my back. It was a gorilla, never mind a monkey!
"He played absolutely brilliantly. He did my head in by missing a few short putts, but he didn't need to win by four, did he? One was good enough."
Although a revered caddy, Foster has been left frustrated by numerous near-misses and partnerships with several elite golfers failed to yield a major.
He linked up with Fitzpatrick in 2018 and was overcome by emotion when Will Zalatoris missed his final birdie attempt to give the pair their first major together.
Foster covered his face with his hat and looked down at the ground before being embraced by Fitzpatrick.
He said: "It means a lot. Obviously, Westy [Westwood], Darren Clarke, Seve, they had their chances over the years, Thomas Bjorn I was caddying for that day when he left it in the bunker at Sandwich [2003 Open] and that really hurt - I thought about it every day for six months, it broke my heart.
"But that has put a lot of bad memories to bed. It means everything to me."
Cameras also captured Foster kissing the flag on the 18th hole to celebrate finally securing the major which had eluded him.
After the triumph, Fitzpatrick revealed Foster had assured him his time would come and the encouraging words were proven true.
Fitzpatrick told Sky Sports: "Billy had been saying for a while, 'the time will come, you're playing so well, just keep doing what you're doing, it will come, it will happen'.
"I put myself in position after two rounds and then played well yesterday. I just really believed this could be the time.
"For whatever reason, because of my success here before, it just felt like this was the time."
Foster may have dedicated years to golf but is also an avid follower of Leeds United and in April 2019, was pictured wearing a Whites shirt whilst in Augusta for the Masters tournament.
In March 2019, he donned a Leeds hat at the Arnold Palmer Invitational.
Various figures from the sporting world, such as football legend Gary Lineker and golf icon Shane Lowry, have taken to social media to praise Foster and the official Leeds United social media accounts have also congratulated him.
Lineker said: "So pleased for Billy Foster. Great guy. Great caddy. Deserves a major."
Lowry tweeted: "So happy for Fitzy and his family...and Billy Foster. What a legend of the game. He deserved this one. Enjoy the celebrations lads."