[1][2][3] Grantham defeated Matt Heimerich in the 2010 Republican primary for a seat in the Colorado Senate from the 2nd district, and won against Democratic nominee Gloria Stultz in the general election.

[10] Grantham was criticized for his handling of sexual assault and harassment claims against member of the state senate due to partisanship.

He stated that Senator Daniel Kagan had acted improperly before an investigation was started while he stated that investigations into Republican senators Randy Baumgardner, Larry Crowder, and Jack Tate were flawed.

[11] He requested for Beth McCann, the Denver District Attorney, to start investigations on all sexual harassments complaints filed against state legislators.

Grantham announced on November 14, 2019, that he would run for a seat on the Fremont County Commission from the 1st district and was elected without opposition.

[1][2][15] During the 2022 United States Senate election he organized Eli Bremer's campaign in Fremont County.

[16] Grantham stated that regulations should be enacted on the construction of mosques after a speech by Geert Wilders.

[18] He introduced legislation to allow business owners to use lethal force if they believed that their property was threatened.