tristan o'tierneyTristan O’Tierney has left Square to circle the globe.

O’Tierney was one of the founders of Square, which is now one of Silicon Valley’s hottest tech companies. O’Tierney cofounded the company in 2009 with Twitter cofounder Jack Dorsey to make it easier for microbusiness owners to accept credit card payments.

Last night, he tweeted that it was his last day at Square after 4 years.

O’Tierney graduated from the Rochester Institute of Technology in 2008 with a degree in computer science. He worked as a Mac engineer at Yahoo, Apple, and VMware and created the official Obama 2008 iPhone app. He built one of the first Twitter client apps on the App Store called Twinkle while working as an iOS developer for video game company Tapulous.

He and Jack Dorsey met at iPhoneDevCamp in 2008. Shortly after, Dorsey left Twitter, and the two began mulling over the concept for Square. While Dorsey worked on the business side and the Square reader, O’Tierney worked on the iOS applications to go with it.

The company has since grown by leaps and bounds. It has raised $341 million and is leading the charge to transform the point-of-sale (POS) industry. Square has expanded its offering to cater to businesses of all sizes — from a food truck owner to national corporations — and Square Register now involves a full suite of software with checkout customization features, management tools, and data analytics.

Square also released the latest version of its consumer app Square Wallet last week to make mobile checkout even easier. It recently launched in Japan and Canada and has spawned international clones like iZettle, Payleven, and mPowa.

The business side of the company is by all signs doing well. However, a number of Square executives have departed this year, including chief operating officer Keith Rabois, who departed in January after being accused of sexually harassing an employee. Then in May, AllThingsD reported that two more executives were leaving. Alex Petrov, who Square hired just three weeks before to serve in the new vice president of partnership position, never ended up joining the company, and Square’s vice president of international Alyssa Cutright confirmed that she left in March.

AllThingsD speculated that these moves could be connected to Square’s hire of ex-Googler Francoise Brougher as business lead.

The circumstances around all these departures (except for Rabois) are still under wraps. Also vague are O’Tierney’s future plans, which he said are uncertain except for “a bit of traveling.”

Photo Credit: kballard/Flickr