Grantham's family owned a cattle ranch, and his father was the founder and CEO of an aviation business named International Air Response, Inc. (IAR).

During his time at Arizona State, he enlisted in the Air National Guard, ultimately achieving the rank of staff sergeant before commissioning as an officer out of college.

[3] In 2016, Grantham and incumbent Eddie Farnsworth defeated LaCinda Lewis in the Republican primary for Arizona's 12th legislative district.

[4] In 2018 Grantham sponsored a proposed state constitutional amendment which would have allowed the legislature to place candidates for the US Senate on the ballot.

[7] In April 2021, he made news after referring "colored people" and demanding that a black legislator be "sat down" during a debate about a voting rights bill.