Gates — who has an estimated net worth of US$102.5 billion ($154.2b) — said that failure was worth a fortune.
"There's room for exactly one non-Apple operating system and what's that worth? US$400b ($601.6b) that would be transferred from company G to company M," The Verge reported Gates as saying.
It's not the first time Microsoft's Android error has been discussed, however it has widely been attributed to former Microsoft CEO Steve Ballmer, who held the top job when the iPhone was first released.
At the time, he was a vocal critic of the new technology, famously criticising it as the "most expensive phone in the world" that would not "appeal to business customers" as it lacked a keyboard.
At that time, Microsoft was persisting with Windows Phone, which failed to embrace the touch-only technology that ended up changing mobile phones forever.
Google snapped up Android for US$50 million back in 2005, and it is now Apple's only serious competitor.
Gates said while Microsoft continued to be a successful company, that misstep stopped it from becoming a global leader.
"It's amazing to me that having made one of the biggest mistakes of all time, that our other assets Windows and Office are still very strong," he said.
"So we are a leading company, but if we had gotten that one right, we would be the leading company."
Meanwhile, Ballmer also dished on his biggest professional snafu shortly before leaving Microsoft in 2014.
He said operating system Windows Vista was his biggest regret.
"I would say probably the thing I regret most is the, what shall I call it, the loopedy-loo that we did that was sort of Longhorn to Vista," he said in an interview with ZDNet.
"I would say that's probably the thing I regret most."
He was referring to Microsoft's decision to reset Vista's codebase in the middle of its development, which meant some of its features that had been publicly promised were scrapped.
"We tried too big a task and in the process wound up losing thousands of man hours of innovation," he told the Seattle Times in 2010.
Bill's billions
Microsoft's failure to take on Apple hasn't hurt Gates' personal fortune.
Today he's the second richest person in the world, just behind Amazon founder Jeff Bezos, who has a net worth of US$154.4b.
Over the years, he and wife Melinda have emerged as leading philanthropists, famously pledging to give away at least half of their wealth after signing the Giving Pledge with investor Warren Buffett in 2010.
The pledge encourages the world's elite to donate much of their wealth to worthy causes, and since then some of the planet's biggest names have also joined the ranks.
