Walter Washington De Lacy was born in Petersburg, Virginia, on February 22, 1819, the son of William and Eliza De Lacy. At the age of fifteen he entered Mount St. Mary's Catholic College in Emmetsburg, Maryland, and graduated in 1838. The following year De Lacy sought and received an appointment to West Point, but his appointment was revoked due to a clerical error. However, De Lacy enrolled at West Point for private instructions in mathematics, surveying, civil engineering and topography. Following the completion of this tutoring he started his career as a civil engineer at the age of twenty. For several months in 1839, De Lacy was employed as an engineer for the Illinois Central and the Iron Mountain Railroad. De Lacy was then commissioned as a captain in the U.S. Army, but resigned before joining his regiment. He devoted the next five years to teaching modern languages to midshipmen onboard the vessels, Pennsylvania, Marion, and Ohio. After various engineering enterprises throughout the Southwest, De Lacy joined Colonel John Mullan in 1859, to survey and build a military road from Fort Benton, in what is now Montana, to Walla Walla, Washington Territory. De Lacy was discharged in 1860, and for the next four years he prospected in the Pacific Northwest. In 1864 he laid out the townsite of Fort Benton. He attended the First Montana Territorial Legislative Assembly in Bannack that same year. The Assembly directed De Lacy to draw the first map of Montana Territory for the purpose of laying out counties. In 1865 De Lacy platted the townsites of Deer Lodge and Argenta. By 1867, during the Sioux War, he had been appointed Colonel of Engineers by Acting Governor Thomas F. Meagher. From 1867 to 1871 he was engaged in the office of the Surveyor-General making maps for the Northern Pacific Railroad. He was elected City Engineer of Helena in 1883, and later served as Chief Clerk to the U.S. Surveyor General until his death on May 13, 1892. De Lacy was a charter member of the Montana Society of Civil Engineers, a member of the Montana Society of Pioneers, and a corporate member of the Historical Society of Montana.