But Mr. Blomfield — who sat for an interview at his London apartment after walking the office dog, a cockapoo named Bingo — argues that day-to-day retail banking has been relatively unaffected.

Monzo, like other start-up banks, still has to figure out how to be profitable. But it has already begun to reimagine banking for an increasingly cashless society. Customers of the future won’t need branches when they can text a customer service representative. Detailed spending breakdowns help account holders control their spending. Doing without retail space and tellers keeps costs down, allowing the company to reduce fees and hire programmers.

“The internet lets you run these traditional businesses at a fraction of the cost,” Mr. Blomfield said.

The company sends celebratory GIFs on Twitter to new customers, and offers a bright coral-color A.T.M. card that is Instagram-ready. Users can visit the office to see what’s being worked on, test new features and perhaps even pet Bingo. Or they can view the company’s product road map, published online.



Monzo got started with crowdfunding and remains a minnow compared with the giants of British banking, but customers — notably younger, wealthier ones — are signing up. Roughly 75 percent of Monzo’s 900,000 clients are under 40, split evenly between men and women. On average, they make more than 50,000 pounds, or about $65,000, a year, nearly double the median British salary, and Monzo is adding more than 2,000 customers a day.

Digital banks face innumerable issues, though. Chief among them is how they will make money. A slick app and basic checking accounts are unlikely to be enough, unless they can offer services like mortgages and other loans that come with higher interest rates.