The start up said that 500 Amex card holders spent £1m via the service in a few months.
Smart credit service Curve said that Amex has joined its app, which lets shoppers hold all their credit and debit cards on a single card.
It added that since this decisive stage of testing began last November, around 500 Curve users have spent more than £1m on their Amex cards, paying through the all-in-one service.
The app said bringing Amex onto its platform also boosts the number of outlets that accept the US charge card, because for the first time it will be approved at any business that uses Mastercard.
Diego Rivas, lead product at Curve, said this move along with other items such as eliminating currency conversion fees, mean that the app is “supercharged with features and services that can only be found today with challenger banks”.
It has more than 100,000 customers who actively use the card, spending on average £1,500 each month, according to the start up.