Cal Henderson

Cal Henderson didn’t set out to build Slack. In fact, he and his co-founders were making a multiplayer online game. But when that game flopped, it wasn’t the fantasy world they’d built that survived – it was the internal messaging tool they’d hacked together to work better as a team. That prototype became Slack: the $26bn company redefining how millions of people work and communicate daily.

Born in Bedford, UK, in 1981, Cal grew up with an early passion for computers, coding his first programs before he hit his teens. Colour-blind and creative, he found joy in building tools that worked better – not just for himself, but for others too. He studied software engineering at Birmingham City University, and by the time he graduated, he was already immersed in open-source communities and internet culture. He co-ran B3ta, a British digital collective known for its subversive humour, and maintained a blog where he shared experiments, code, and commentary under the moniker iamcal.

His career took a defining turn in 2003 when he joined Flickr’s founding team as its first web developer. There, he helped scale the platform during a period of explosive growth and played a lead role in developing what became one of the earliest modern web applications. That experience – designing for scale, speed and usability – laid the technical foundation for what came next.

In 2009, Cal reunited with Flickr co-founder Stewart Butterfield to build Tiny Speck, a company aimed at creating an online game called Glitch. While the game never found a lasting audience, the internal messaging tool the team used for collaboration became the unexpected success story. It offered faster, more structured communication than email – and resonated with early testers outside the company. Cal and the team realised the tool had broader potential, and in 2013, Slack was launched publicly.

The product gained traction quickly, reaching 500,000 daily users in its first year. Cal, as CTO, led the effort to scale the platform without sacrificing simplicity. By 2020, Slack had surpassed 12 million daily users and been acquired by Salesforce for $27.7bn – one of the largest software deals in tech history.

Inside Slack, Cal built a culture that prized technical pragmatism and clarity. He has spoken publicly about the importance of making software development efficient and resilient, while avoiding unnecessary complexity. Rather than chasing architectural perfection, his team focused on building scalable systems that could be adapted as real-world usage evolved.

Beyond Slack, Cal’s personal projects reflect his curiosity and craft-first mindset. He authored Building Scalable Web Sites early in his career, contributed to the development of OAuth, and created numerous open-source tools ranging from Unicode character utilities to lightweight JavaScript libraries. His long-running personal site, iamcal.com, remains a trove of software experiments and internet culture artefacts.

Cal doesn’t fit the typical Silicon Valley founder archetype. He never learned to drive, has spoken candidly about the unglamorous side of leadership, and brings a sharp eye to hype-driven tech narratives. Yet his impact is undeniable. From an abandoned video game to the backbone of enterprise collaboration, his story is a rare example of quiet engineering excellence leading to global scale – not by chasing the next big thing, but by recognising when something already built just works better.