His grandfather, George Theodore Mickelson was the 18th Governor of South Dakota.

George Mark Mickelson married Cynthia Hart on August 31, 1996, and has three sons.

When incumbent Representatives Democrat Susy Blake and Republican Brian Liss left the legislature and left both the District 13 seats open, Mickelson ran in the three-way June 5, 2012, Republican Primary and placed first with 2,063 votes (53.6%); Mickelson and fellow Republican nominee Steven Westra were unopposed for the November 6, 2012, general election, where Mickelson took the first seat with 7,122 votes (58.06%) and Westra took the second seat.

[4] Mickelson served as the South Dakota Speaker of the House Representatives like his father and grandfather before him.

Mickelson decided to retire from politics in 2018, after choosing not to run for Governor of South Dakota, again an office his father and grandfather previously held.