Five years ago, Facebook turned down Brian Acton for a job.

The Orlando, Fla.-raised software engineer had worked at Yahoo YHOO for over a decade when he decided to take time off. For two years he did, exploring places as far-flung as Antarctica before returning to Silicon Valley to work again. After companies like Facebook FB and Twitter TWTR rejected him, he started building WhatsApp, a mobile messaging service that eventually exploded, amassing 500 million users worldwide. Then last February, Facebook stunned the world when it announced it was scooping up WhatsApp for a jaw-dropping $19 billion – the most it had ever paid for a startup. By one estimate, Acton will be worth at least $3 billion when the deal closes, which is expected later this year.

That Facebook once rejected Acton, 42 is an irony not lost on him. But far from being bitter he says he looks forward to working with Facebook CEO Mark Zuckerberg and crew. “We might disagree on some topics, but they understand what communication is like, and they understand the issues around privacy and security,” Acton explained Wednesday at StartX, a non-profit organization for Stanford University entrepreneurs.

Reaching a $19 billion deal was a roundabout journey for Acton and Koum. For one, Acton was already 38 when he helped build WhatsApp. Before that, he spent three years at Apple AAPL and over 11 years at Yahoo, where he met WhatsApp CEO and cofounder Jan Koum and eventually became the company’s vice president of engineering. (Acton also weathered a divorce and had children.) It’s a different — and far longer — trajectory compared to many of today’s entrepreneurs, who jump into a startup right after college or drop out, Acton points out.

Still, Acton and Koum’s late-blooming strategy worked. With WhatsApp, they developed a dead-simple mobile app that works a lot like traditional text, or SMS messaging, allowing users to send and receive calls, video, and pictures in addition to messages. (“I used to call SMS black and white,” Acton said. “We’re color.”) Because it was free, the app developed a huge following, particularly in Europe and parts of Asia, where traditional texting can be pricey. That’s despite a mobile messaging space crowded with competitors like Line, Viber and MessageMe. “It just effing works,” said Acton, explaining in semi-profane terms WhatApp’s appeal. “We don’t have a lot of gimmickry. We don’t collect messages or do anything with them. We respect our users.”

Talks with Zuckerberg about a potential WhatsApp acquisition began heating up in early February, when Zuck – as he is known to tech insiders – presented Acton and Koum with a hard number. “We said, ‘Oh, shit,’ We’ve got to pay attention to this,” Acton said, who recalled a mind-numbing 96-hours straight in conference rooms with a “flotilla” of lawyers as they hammered out a deal.

For now, the thing Acton looks forward to most isn’t working with the employer who once rejected him – or even getting to 600 million WhatsApp users – it’s closing the deal with Facebook. Admitted Acton: “When it closes, it’ll be with a sense of relief.