More and more banking apps are adding support for Android 10's new biometrics API that allows you to use the Pixel 4's face unlock mechanism to access your account, mitigating the need to fill in your password over and over again. Ally Bank is the latest financial institution to join this growing group. The newest version of the Ally Mobile app now supports both fingerprint and face unlock, depending on your device.
When you first start the app after updating it to version 184.108.40.206, you should be greeted by a prompt that asks you to enable biometric unlock of the app. If it doesn't show up, you can always head to Ally Mobile's settings to activate it.
The capability is live in version 220.127.116.11 of the app, which is currently rolling out on the Play Store. If it's not available for you yet, you'll need to be patient — it'll take a while until it reaches all customers.