In December 1913, Hardwick was engaged to Margaret Stone, the daughter of financier Galen Stone of the brokerage firm of Hayden Stone & Co. Stone was a debutante in 1913 and an "enthusiastic follower" of Harvard football. The couple married in 1915, and their wedding, attended by 1,000 guests, was described as "a brilliant social event," and "the most sumptuous bridal that ever graced the Buzzard's Bay shore." After a wedding breakfast, the couple left in an automobile for a honeymoon trip. The couple had one child, Margaret, born in 1917.