He was born at Cold Spring, N. Y. in 1846. Mabie was the youngest child of Sarah Colwell Mabie who was from a wealthy Scottish-English family and Levi Jeremiah Mabie, whose ancestors were Scots-Dutch. They were early immigrants to New Amsterdam, New Netherland about 1647. Due to business opportunities with the opening of the Erie Canal his family moved to Buffalo, New York when he was approaching school age. At the young age of 16 he passed his college entrance examination, but waited a year before he attended Williams College (1867) and the Columbia Law School (1869).