Peter A.S. McGlashan was born in Edinburgh, Scotland on May 19, 1831. He was the son of James McGlashan, a veteran of the Napoleonic Wars and Waterloo. His grandfather, namesake Peter McGlashan, was the last chief of Clan McGlashan (now a sept of Clan McIntosh). The McGlashan family immigrated to Savannah, Georgia in 1848. A year later eighteen years old Peter participated in the California Gold Rush, but returned without much success. In 1856 he joined Walker's army and fought in the Filibuster War in Nicaragua. When the war ended he returned and settled in Thomasville, Georgia; opening a store.