Barno was commissioned a second lieutenant of infantry upon graduation from the United States Military Academy at West Point, New York, with the class of June 1976.

In 1988, Barno joined the 2nd Ranger Battalion as the S-3 (Operations) officer where he played a key role in the planning and execution of the subsequent invasion of Panama.

Barno was the principal author of the Army's transformational 2003 Warrior Ethos which dictates: "Mission First, Never Accept Defeat, Never Quit, and Never Leave Behind a Fallen Comrade."

Ambassador Zalmay Khalilzad, co-locating his command at the US Embassy compound and creating an integrated civil-military Counter-insurgency campaign plan for Afghanistan focused on the Afghan population.

Following his retirement from active duty, Barno served as the director of the Near East South Asia Center for Strategic Studies (NESA) at the National Defense University in Washington, D.C. from 2006 until April 2010.

Barno also travelled widely throughout the Middle East and South Asia region in his NESA director capacity, and lectured at Harvard, Yale, Tufts, Johns Hopkins SAIS, West Point, and the US Army and Naval War Colleges.

He also served as a Non-Resident Senior Fellow at the Atlantic Council from May 2015 to March 2018, where he co-authored with Dr. Bensahel a major report in 2016 entitled The Future of the Army.

Barno and Bensahel published their first book, Adaptation Under Fire: How Militaries Change in Wartime at Oxford University Press in September 2020.

Barno's articles, book reviews and opinion pieces have appeared in Foreign Policy, The Atlantic,[6] the Washington Post, and numerous military journals.

He has appeared in interviews for a wide range of television and radio outlets, including CNN, BBC World, PBS NewsHour, C-SPan's Washington Journal, Al Jazeera, Fox News, Government Matters and NPR.