The son of a wealthy old Dutchess County family, de Peyster joined the Union Army at the age of eighteen.

He saw service in the eastern theater, and is best remembered for raising the first Union flag over the Confederate capitol of Richmond, Virginia, after its fall in 1865.

In 1900, the family feud culminated in a race for the office of Mayor of their native town, father running against son.

[4] At the start of the war in 1861, Johnston was fifteen years old and attending Highland Military College in Newburg, New York.

De Peyster's father paid ten-dollar bounties to the men who signed the muster roll of his company.

[7] The division was part of Major General Benjamin Butler's disastrous Bermuda Hundred Campaign and was engaged at Swift Creek on May 9.

He was stationed at Fort O'Rourke located in Norfolk, Virginia, where he contracted a fever and was sent home for six months, missing the unit's heaviest engagements at the siege of Petersburg and the second battle of Fair Oaks.

[1] Upon his return, he joined the staff of Brigadier General George F. Shepley, commanding the military district of Norfolk.

Admiral David Farragut later claimed that de Peyster was due as much credit as he would have received had he actually taken the city by storming it, because doing it without bloodshed still carried the same intent.

[18] The young lieutenant reflected on the incident in a letter to his mother:[19] Headquarters, Army of the James Richmond, April 3, 1865.

JOHNSTONFor this act, he received a brevet promotion to lieutenant colonel of volunteers, "for gallant and meritorious conduct, and for hoisting the first real American flag over Richmond, Va., after its capture by the Union forces, April 3, 1865, and as a testimonial of the zeal, fidelity, and courage with which he had maintained the honor of the State of New York in her efforts to enforce the laws of the United States, supremacy of the constitution, and a republican form of government".

He also joined the Military Order of the Loyal Legion of the United States and the General Society of Colonial Wars.

[27] Another incident occurred and was reported in the press when Johnston's mother fell ill, and he visited her at the de Peyster family home and was violently assaulted by his father.

[3] He was married to Julia Anna "Annie" Toler and had three daughters:[3] De Peyster died in 1903, predeceasing his father, as did all of his siblings.