The software engineer who reverse-engineered his school’s systems in middle school, led Poland’s top League of Legends organization, and now builds autonomous AI at a Khosla-backed startup.
Code Before Algebra
Mateusz Kelner built his first website at eleven years old. By middle school, he had reverse-engineered his school’s student registry application, not to cause trouble, but because he wanted to understand how the system worked. By high school, he had founded his first startup. Before he was old enough to vote, Kelner had competed in Poland’s national programming Olympiad finals, placing among the top student programmers in the country, and had collected scholarships from Credit Suisse, the local government, and his high school for achievements in science and technology.
The Credit Suisse Quant Scholarship he received at Wroclaw University of Science and Technology was particularly notable: the programme had an approximately one percent acceptance rate and was typically awarded to students in their third or fourth year. Kelner received it during his first year. After completing that year, he chose to pursue software engineering full time, a decision that marked the beginning of a career that would compress a decade’s worth of building into just a few years.
Professional Engineering at Eighteen
Kelner secured his first professional software engineering position in 2021, a few months before turning nineteen. At BigShortBets, a Poland-based technology company, he built a social media platform from scratch using React, Django, and PostgreSQL, scaling it to over ten thousand users. He also implemented the frontend of a decentralized peer-to-peer futures trading market, gaining exposure to the intersection of financial technology and distributed systems that would shape his subsequent work. The role was not an internship or a junior rotation; it was a full-stack engineering position where Kelner was trusted to architect and deliver production systems at an age when most of his peers were completing their first year of university coursework.
He subsequently joined Purplemana as a software engineer and quickly became the frontend lead for an application with approximately one thousand monthly active users. It was here that Kelner engineered one of his most technically notable contributions: a card recognition feature that achieved a seven-times speed improvement over the previous implementation. By deploying a pretrained YOLO object detection model combined with perceptual hashing directly on the client side, he eliminated server round-trip latency entirely, an approach that was novel in the collectible card technology space and demonstrated his ability to bring machine learning techniques into practical product contexts.
Leading Poland’s Top League of Legends Organization
In 2022, while simultaneously working as a software engineer, Kelner co-founded an esports organization that would become the most popular League of Legends organization in Poland. As CEO, he raised approximately $50,000 in angel funding, built a following of thousands across Twitter, TikTok, and YouTube, and led the organization to receive the award for Best League of Legends Organization in Poland in 2023, a recognized distinction within the country’s competitive gaming community.
The organization’s rise attracted significant media attention. Kelner appeared in a television broadcast interview on one of Poland’s top three broadcasters’ gaming-focused channels, discussing the organization’s origins and future plans. The esports venture was covered in the press numerous times during its growth period. While the organization ultimately did not succeed as a long term business, it demonstrated Kelner’s ability to build, lead, and scale operations far beyond the scope of individual engineering contributions, managing teams, investors, media relationships, and community engagement simultaneously, all before the age of twenty two.
Serial Entrepreneurship Across Two Continents
Kelner’s entrepreneurial output accelerated from there. In 2023, he co-founded MagicTown, a Polish collectibles trading company buying and selling Magic: The Gathering cards worldwide. The company grew from zero to $250,000 in yearly revenue, achieved profitability, and continues to operate and grow under the co-founders to whom Kelner sold his majority stake in November 2024. In early 2024, he co-founded PMX Labs Inc., a U.S.-incorporated company that built a proprietary digital booster pack marketplace for collectible cards. Kelner personally designed and built the Consumer facing side of the product, including two custom algorithms for dynamic pricing and order splitting, and grew the business from zero to a millions in annual revenue run rate, with monthly revenue climbing into the hundreds of thousands and monthly profit reaching the tens of thousands by mid-2025, all without external funding.
The combined picture is striking: by age twenty four, Kelner had founded or co-founded four companies across two countries, built multiple products from scratch that reached thousands of users, generated millions in revenue, hired and managed teams, and appeared on television, all while maintaining the hands on engineering capability to personally architect and write the code that powered his businesses.
Now Engineering the Future of Autonomous Accounting
In November 2025, Kelner joined Synthetic Intelligences Inc. as a Product Engineer. The startup, founded by Ian Crosby (the founder of Bench Accounting, North America’s largest small business bookkeeping service) and backed by Khosla Ventures, is building an autonomous AI bookkeeper that processes financial data, manages bookkeeping workflows, and communicates with customers without human intervention. Kelner is the second engineering hire, responsible for building the core product infrastructure from the ground up: data ingestion pipelines connecting to banks, financial statement extraction engines, and the AI bookkeeper’s transaction categorization system.
For a builder who has spent his career turning ideas into working systems at extraordinary speed, the move to autonomous AI represents both a natural evolution and a dramatic increase in scale. The small business bookkeeping market he is now helping to transform has the potential to affect millions of businesses. The technology he is building could fundamentally change how those businesses manage their finances. And the pattern remains the same as it was when he built that first website at eleven: identify a problem, understand the system, and build something that works.