Austin was commissioned as a second lieutenant after graduation from West Point. His initial assignment was to the 3rd Infantry Division (Mechanized) in Germany where he served as a Rifle Platoon Leader in A Company, 1st Battalion, 7th Infantry and Scout Platoon Leader in the Combat Support Company, 1st Battalion, 7th Infantry. Following this assignment and attendance at the Infantry Officer Advanced Course, he was assigned to the 82nd Airborne Division at Fort Bragg, North Carolina, where he commanded the Combat Support Company, 2nd Battalion (Airborne), 508th Infantry and served as the Assistant S-3 (Operations) for 1st Brigade, 82nd Airborne Division.
In 1981, Austin was assigned to Indianapolis, Indiana where he served as the Operations Officer for the U.S. Army Indianapolis District Recruiting Command and later commanded a company in the U.S. Army Recruiting Battalion. Upon completing this assignment, he attended Auburn University where he completed studies for a Master's Degree in Education. He was then assigned to the U.S. Military Academy at West Point, New York, where he served as a Company Tactical Officer. After his selection and subsequent completion of the U.S. Army Command and General Staff College, Fort Leavenworth, Kansas, he was assigned to the 10th Mountain Division (Light Infantry), Fort Drum, New York, where he served as the S-3 (Operations) and later Executive Officer for the 2nd Battalion, 22nd Infantry. He subsequently served as Executive Officer for 1st Brigade, 10th Mountain Division (Light Infantry) and later as Director of Plans, Training, Mobilization and Security for Fort Drum, New York.
In 1993, Austin returned to the 82nd Airborne Division at Fort Bragg, North Carolina where he commanded the 2nd Battalion, 505th Parachute Infantry Regiment. He later served as the G-3 for the 82nd Airborne Division. Following graduation from the U.S. Army War College at Carlisle Barracks, Pennsylvania, he commanded the 3rd Brigade, 82nd Airborne Division at Fort Bragg, North Carolina. Shortly after Brigade command, he was assigned to the Pentagon in Washington, D.C. where he served as Chief, Joint Operations Division, J-3, on the Joint Staff. His next assignment was as Assistant Division Commander for Maneuver, 3rd Infantry Division (Mechanized), Fort Stewart, Georgia. As the ADC-M, he helped spearhead the division's invasion of Iraq in March 2003.
Austin served from September 2003 until August 2005 as the Commanding General of the 10th Mountain Division (Light Infantry), with duty as Commander, Combined Joint Task Force-180, Operation Enduring Freedom, Afghanistan. His next position was Chief of Staff of the United States Central Command at MacDill AFB, in Tampa, Florida from September 2005 until October 2006.
On December 8, 2006, Austin was promoted to Lieutenant General, and assumed command of the XVIII Airborne Corps, Fort Bragg, North Carolina. Austin handed over command of XVIII Corps to become Director of the Joint Staff in August 2009.
In February 2008, Austin became the second highest ranking commander in Iraq, taking command of the Multi-National Corps-Iraq (MNC-I). As commander of MNC-I, he directed the operations of approximately 152,000 joint and coalition forces in all sectors of Iraq.