MasterCard customer service representatives provide assistance for customers in the United States at 800-627-8372, according to the company's official website. At MasterCard.us, customers can also complete a web form to receive assistance via email and find answers to hundreds of the most frequently asked questions.
MasterCard.us provides a list of toll-free numbers in countries around the globe that customers use to find assistance while traveling, according to the company's official website. MasterCard also interacts with customers and potential cardholders via Facebook, Twitter, YouTube and LinkedIn. To report a lost or stolen card, contact the toll-free MasterCard assistance line, and notify the financial institution that issued the credit card, according to MasterCard.us.