McMaster was sworn in as governor of South Carolina on January 24, 2017, minutes after former Governor Nikki Haley's confirmation vote as the U.S. ambassador to the United Nations. He will serve out the remainder of Haley's term, which expires in January 2019.

McMaster was first elected lieutenant governor in 2014. He was inaugurated the following January for a four-year term in office.

McMaster was first elected to the attorney general position in 2002 and won a second term in 2006. He opted not to seek re-election in 2010 in order to pursue a bid for governor, which was not successful. He left office when his second term expired in January 2011.