Bloomberg reported that this gave advertisers on the search engine the ability to find out if the ads they ran had led to a sale at a bricks and mortar store or not.
Mastercard holders, some two billion globally, were unaware of this because neither of the companies have made it public, the report said.
The news wire cited four people who were aware of the deal as saying that Google and Mastercard had negotiated the deal over four years. It gave Google a means of gauging retail spending, data which it can use to plan its strategy against competitors like retail behemoth Amazon.
Google was fined €4.34 billion (US$5.05 billion, A$6.82 billion) in July for breaching anti-trust rules over its Android mobile operating system.
Last year, it was fined €2.42 billion (US$2.7 billion) for allegedly abusing its search engine dominance to give illegal advantage to its own comparison shopping service. The company has appealed the decision.
A third fine is said to be in the EU pipeline, this for alleged anti-business practices involving Google's AdSense advertising system.
The report quoted Christine Bannan, counsel with the Electronic Privacy Information Centre, as saying: "People don’t expect what they buy physically in a store to be linked to what they are buying online.
"There’s just far too much burden that companies place on consumers and not enough responsibility being taken by companies to inform users what they’re doing and what rights they have.”
The sources who spoke to Bloomberg said Google had paid millions of dollars for the data.
The report said Google denied having a revenue-sharing deal with its partners and was unwilling to comment on the partnership with Mastercard.
It said people could opt out of ad tracking by using its "Web and App" activity configuration panel.
Mastercard also refused to comment on any deal with Google.