Prior to leading INSCOM, he held various roles in Army intelligence supporting U.S. operations in primarily Southwest Asia.

In his 34-year career in the United States Army, Johnston held command at every level from company to two-star general.

Early in his career, he served in Operation Desert Storm as a squadron intelligence officer in the 3rd Armored Cavalry (1990–1991).

Returning to the US, Johnston was assigned as G-2 of the XVIII Airborne Corps (2012–2015), before becoming J-2 at U.S. Special Operations Command (USSOCOM) from 2016 to 2017.

He was married to Army Brigadier General Amy E. Johnston (née Hannah), a public affairs officer whose tenure as the Army's Chief of Public Affairs was mired in controversy over her handling of the murder of specialist Vanessa Guillén,[9] and a survey of her leadership climate in which 97% of subordinates described as hostile, including concerns of sexual and racial harassment.