Mars married Virginia Cretella, born 1929/1930 (age 87–88). They had four children: Victoria B. Mars, Valerie Anne Mars, Pamela Diane Mars, and Marijke Elizabeth Mars.
Mars graduated from the Hotchkiss School in Lakeville, Connecticut, in 1949 and Yale University.
He divorced Virginia in 1990 to marry Deborah Adair Clarke. They divorced in 2010.
Mars died at age 84 on July 26, 2016, in Seattle, Washington, of complications from a heart attack.
Mars and his ex-wife donated the funds that made it possible for the Colonial Williamsburg Foundation to reconstruct an 18th-century coffeehouse in Colonial Williamsburg.
As owner of the Diamond Cross Ranch, an 82,000-acre (33,000 ha) parcel along Montana’s Tongue River and on the northern end of the Powder River Basin, Mars was active in opposing the development of his part of what's been called the "most productive coal and natural gas fields in the nation." Companies that hold the oil and gas leases to his land, rights originally made possible by the Stock-Raising Homestead Act and the Mineral Leasing Act, are seeking to exercise those rights on his ranch. Mars was reportedly concerned about the large amount of water that Energy exploration and production projects consume, water needed by his ranch.