Robert Mills Grant learned cowboying and ranching on the ranch his Scottish grandparents Robert and Margert Grant established in 1884. He and his parents, Duncan and Cora Grant had a cow/calf operation.
As a youngster he rode his horse to school on the CMD Ranch on Richeau Creek half the year and the other half went to school in the rock school house built on the Grant Ranch. He was graduated from high school in Wheatland.
He continued to own and operate the family ranch. He served 16 years on the Platte County Fair Board and helped with the PRCA sanctioned rodeo held in conjunction with the fair for 15 years. A 60-year member of the Wyoming Stockgrowers, he was Chairman of the Brand and Theft Committee. He also served ten years in the Wyoming State House of Representatives.