How Mollie uses Expo to power its multi-platform payments app

This is a guest post from Vernon de Goede - the Director of Engineering and Domain Lead at Mollie where he drives the strategy and execution of the Merchant Experience domain.

...

In 2019, the story of Mollie and Expo began not in a boardroom, but in a bar in Amsterdam. I met Cedric van Putten from Expo for a few drinks. By the end of the afternoon, I was convinced that Expo was the right platform for Mollie. That same weekend, I built the first version of Mollie’s mobile app using Expo.

Six years later, the results speak for themselves: a thriving, secure, and resilient mobile app used by tens of thousands of customers every day.

At Mollie we needed to build a mobile app that could scale quickly across platforms — and do so with a team that had little to no native mobile experience. Expo offered exactly what we were looking for:

• Speed of delivery: A small mobile team rapidly delivered a customer-ready mobile app within weeks - especially impressive, considering building mobile capabilities from the ground up.
• Stability: Mollie's Expo-powered app is now more resilient than many native counterparts — bug fixes can be deployed instantly without needing to wait for store reviews.
• Developer experience: With Expo’s transparent engineering culture and excellent documentation, engineers at Mollie were able to onboard quickly and focus on shipping value.
• Trusted relationship: Since 2019, Expo has maintained a direct Slack channel with the Mollie team. We understand that we got a bit lucky to have that. At the time Expo gave it to us because they "thought we were cool" but now it's an enterprise feature.

At Mollie, everyone is a full-stack engineer. Out of 350 engineers, only about 10 had prior React Native experience. The rest? They don’t even realize they’re using React Native (well, until they read this blog...) — they’re simply building features using a cross-platform component library (Mollie UI) that abstracts away the mobile-specific details.

The architecture makes this possible:

• A shared monorepo containing all feature code.
• A core team of 5 engineers maintaining both mobile and web platforms.
• A modern release process that reduces the gap between mobile and web deployments.
• Feature teams free to focus on business logic, without worrying about mobile-specific tooling or constraints.

Over time, some engineers upskilled themselves to build custom Expo modules — enabling advanced features like Apple Tap to Pay, used by tens of thousands daily to send and receive payments.

• React Navigation powers all navigation, although engineers work with abstractions defined in the UI library which control how screens and animations are happening.
• A cross-platform API library that provides seamless requests independent of the platform.
• A custom local authentication library handles secure actions like transfers and approvals.
• Scalable monorepo structure that allow us to grow into more apps, with clear ownership of each cross-platform feature.
• API gateway architecture connects to backend services written in both PHP and Java.
• Advanced security: heavy use of encryption in every API call, especially for sensitive actions like account money movements, MFA, etc.
• Single commands to set up local environments of all platforms and for the creation of new features
• Simplified way to manage data with ReduxJS/toolkit/query.
• Cross sharing code for two apps (one of them for a terminal) and a web platform.
• Expo Modules enable access to native features like Apple Tap to Pay and a safer rooting & jailbreak detection mechanism.

Mollie doesn’t just build apps — we build hardware too. Our terminal device (Tap) runs an Android app also built with Expo. This approach ensures consistency across all our customer touchpoints, from mobile apps to physical terminals.

• Cross-platform velocity: A feature can be built once and deployed to iOS, Android, and web with just ~15% additional overhead. Going fully native would increase that to 300% and require hiring specialists.
• Rapid iteration: Fixes and improvements can be shipped on the fly with OTA updates, reducing operational risk and improving the customer experience.
• Engineering alignment: Expo’s engineering-first philosophy and transparent roadmap align closely with Mollie’s values.
• Scalability: With EAS Build and Expo’s tooling, hundreds of engineers — regardless of mobile expertise — can contribute seamlessly to the app.

Mollie is redefining what’s possible with React Native and Expo. What started as a fast-moving MVP is now a critical product used by tens of thousands to send and receive money securely. With a forward-thinking architecture, empowered engineers, and the right tools, Mollie continues to push the boundaries of what cross-platform apps can achieve.