Siemiatkowski started building Klarna at age 23, while on a sabbatical from the
Stockholm School of Economics. He worked in sales offering
accounts receivable services to small businesses, and noticed significant and regular challenges when processing with e-commerce payments. He co-founded the company with
Niklas Adalberth and Victor Jacobsson in 2005. Siemiatkowski had met one of his co-founders when working at
Burger King. In 2021, he was minted as a billionaire as Klarna's valuation increased to $31 billion. In 2022,
Klarna announced a round of redundancies in response to a global economic slowdown. Departing employees were asked if they wanted to join a voluntary initiative to share their availability on platforms like
LinkedIn. Those who agreed to the initiative had their details shared by Klarna and also Siemiatkowski. By sharing the list, Siemiatkowski subsequently received both praise and criticism in the media. The
Swedish Financial Sector Union expressed regret to have learnt about the job cuts from the media rather than being consulted beforehand and sued the company over the layoffs. In December 2024, he stated that the company would not increase headcount because "AI can do all of the jobs that we humans do. It’s just a question of how we apply it and use it”. Klarna listed on the
New York Stock Exchange in September 2025 at a valuation of $19 billion. At the time, it was the largest IPO of the year on the NYSE. == Personal life ==