Mastercard is a payment network and all payment networks set their own exchange rates that you receive when use your card overseas. The exchange rate is set by the payment network and not by your credit card issuer. Mastercard uses multiple market sources (such as Bloomberg, Central Banks, Reuters and others) to develop exchange rates.
Some credit card issuers can charge you a foreign transaction fee when you make a transaction in a foreign currency. This fee is usually between 1% and 3% per transaction. However, most credit card issuers do not charge this fee nowadays, so it is very easy to find a credit card with no foreign transaction fee. Since there are different payment networks, it means each network has its own exchange rate that may be different. The Mastercard‘s exchange rate is slightly different from Visa‘s exchange rate. Research usually shows that however small the difference may be, Mastercard exchange rate is slightly better than Visa’s. Anyway, mostly your benefit depends on the type of a transaction and on the country you are currently visiting.