[6] Sullivan took part in the Army Reserve Officers' Training Corps while at Norwich and at graduation he received his commission as a second lieutenant of Armor.

[3] During his army career, Sullivan served as: Assistant Commandant, United States Army Armor School at Fort Knox, Kentucky from November 1983 to July 1985; Deputy Commandant, United States Army Command and General Staff College at Fort Leavenworth, Kansas from March 1987 to June 1988; Commanding General, 1st Infantry Division (Mechanized) at Fort Riley, Kansas from June 1988 to July 1989; Deputy Chief of Staff for Operations and Plans; and Vice Chief of Staff of the United States Army from 1990 to 1991.

[3] In August 1993, President Bill Clinton assigned the duties and responsibility of acting Secretary of the Army to Sullivan, who continued to serve as chief of staff.

[3] Sullivan was the co-author, with Michael V. Harper, of Hope Is Not a Method (Random House, 1996), which chronicles the enormous challenges encountered in downsizing and transforming the post-Cold War army.

He was also the President and Chief Executive Officer of the Association of the United States Army, headquartered in Arlington, Virginia from February 1998 through June 2016.

[11] Sullivan was an Advisory Board Member of Spirit of America, a 501(c)(3) organization that supports the safety and success of Americans serving abroad and the local people and partners they seek to help.

[12] In recognition of his military career and his work with AUSA, Sullivan was awarded the prestigious Sylvanus Thayer Award by the United States Military Academy in 2003,[13] and the AUSA General George Catlett Marshall Medal, the Association's highest honor, in October 2016.

[14] Gen. Sullivan served as the first Chairman of the CNA Military Advisory Board,[15] the first group of retired generals and admirals to examine the national security implications of climate change.