Google and Mastercard Cut a Secret Ad Deal to Track Retail Sales

For the past year, select Google advertisers have had access to a potent new tool to track whether the ads they ran online led to a sale at a physical store in the U.S. That insight came thanks in part to a stockpile of Mastercard transactions that Google paid for.

But most of the 2 billion Mastercard holders aren’t aware of this behind-the-scenes tracking. That’s because the companies never told the public about the arrangement.

Alphabet Inc.’s Google and Mastercard Inc. brokered a business partnership during about four years of negotiations, according to four people with knowledge of the deal, ...