In 2017, Alex Mashinsky founded Celsius Network, a borrowing and lending platform for digital assets like Bitcoin, Ethereum, and other cryptocurrencies. It encouraged its customers to "unbank" themselves and offered interest rates as high as 18.6 per cent on cryptocurrency deposits. As CEO, he hosted "Ask Mashinsky Anything," a weekly YouTube livestream in which he answered questions about Celsius. He became known for wearing T-shirts reading "Banks are not your friends" and “Un-Bank Yourself.”

In January 2022, Mashinsky took control of Celsius's trading strategy. Some insiders reported that he personally directed large individual cryptocurrency trades, overruling executives with significant financial experience. Mashinsky withdrew $10 million from Celsius in May 2022. The withdrawals came as customers were withdrawing assets in large quantities while the company was nearing bankruptcy, according to a report in October 2022. A spokesperson for Mashinsky told the Financial Times that the funds were used for tax payments and estate planning. In total, Arkham Intelligence estimates that Mashinsky sold $44 million worth of CEL tokens through exchanges. On July 13, 2022, one month after it paused customer withdrawals, Celsius filed for Chapter 11 bankruptcy. Mashinsky said that "the Company made what, in hindsight, proved to be certain poor asset deployment decisions." Mashinsky resigned as Celsius CEO on September 27, 2022.

On July 13, 2023, Celsius Network agreed to a $4.7 billion settlement with the Federal Trade Commission (FTC). On the same day, Mashinsky was arrested and charged with committing securities, commodities, and wire fraud, and with committing securities manipulation and fraud charges. If they are convicted, Mashinsky and a co-defendant, Roni Cohen-Pavon, face decades in prison. Mashinsky pleaded not guilty to the charges.