A graduate of the United States Military Academy at West Point, New York, where he was ranked 101st out of 103 in the class of 1909, Simpson served in the Philippines, where he participated in suppression of the Moro Rebellion, and in Mexico with the Pancho Villa Expedition in 1916.

Simpson led the Ninth Army in the assault on Brest in September 1944, and the advance to the Roer River in November.

After the war ended, Simpson commanded the Second United States Army, and served in the Office of the Chief of Staff.

His father and uncle had fought with the Confederate Army under Nathan Bedford Forrest in the American Civil War.

On 24 February 1917, he became aide-de-camp to Brigadier General George Bell Jr., the commander of the El Paso Military District.

[19] The citation for his Army DSM reads: The President of the United States of America, authorized by Act of Congress, 9 July 1918, takes pleasure in presenting the Army Distinguished Service Medal to Lieutenant Colonel (Infantry) William Hood Simpson, United States Army, for exceptionally meritorious and distinguished services to the Government of the United States, in a duty of great responsibility during World War I, as Assistant Chief of Staff, 33d Division, during the Meuse-Argonne offensive and later as Chief of Staff of this division.

[20]Upon returning to the United States in May 1919, Simpson was posted to the 6th Division at Camp Grant, Illinois, as its chief of staff from 15 June 1919, to 25 August 1920.

[19] In El Paso, Texas, on Christmas Eve, 1921, he married Ruth Krakauer, an English-born widow whom he had first met while at West Point.

He then attended the United States Army Command and General Staff College at Fort Leavenworth, Kansas, from 15 August 1924, to 19 June 1925, when completed the program of instruction as a distinguished graduate.

Upon graduation, he was assigned to the War Department General Staff in Washington, D.C.,[19] where he worked in the Latin American section of the G-2 branch.

[23] On 20 June 1932, he became Professor of Military Science and Tactics at Pomona College in Claremont, California, and was promoted to lieutenant colonel again from 1 October 1934.

From April to September 1941 he was the first commander of the country's largest Infantry Replacement Training Center, Camp Wolters, located in Mineral Wells, Texas.

He received a promotion to temporary major general on 29 September 1941, and commanded the 35th Infantry Division, an National Guard formation, at Camp Robinson, Arkansas, from 15 October 1941 to 5 April 1942,[26] for which he was awarded the Legion of Merit.

[26] Simpson then commanded the Fourth United States Army from 29 September 1943 to 8 May 1944, with the three-star rank of lieutenant general as of 13 October 1943.

A cadre for the Army headquarters was provided by the Western Defense Command, but all senior officers were approved or selected by Simpson.

The main body of Ninth Army embarked for the UK on the ocean liner RMS Queen Elizabeth on 22 June.

[28][29] Simpson observed the abortive start of Operation Cobra on 24 July with Lieutenant Generals Courtney Hodges and Lewis H. Brereton.

[42] According to Colonel Armistead D. Mead, Simpson's G-3 (Operations) officer: General Simpson's genius lay in his charismatic manner, his command presence, his ability to listen, his unfailing use of his staff to check things out before making decisions, and his way of making all hands feel that they were important to him and to the army...

[40][44] For Operation Grenade, the crossing of the Roer, the Ninth Army was reinforced, its strength increased from five to twelve divisions.

[41] The British Second Army commenced Operation Veritable, the northern part of a pincer movement to clear the Rhineland, on 8 February 1945.

Montgomery's plan was for Operation Grenade, scheduled to commence on 10 February 1945, to form the southern part of the pincer, but there was still no word of the capture of the Roer dams by the First Army.

Montgomery left the decision of whether to delay Operation Grenade up to Simpson, but postponement would make the task of both the British troops already fighting more difficult, and increase the risk that the Germans would detect the Ninth Army's preparations.

Simpson watched the river slowly rise, but could not be certain whether it was the result of German demolition or increased flow due to snow melt.

In view of his brilliant service, it was unfortunate that shortly after the war ill-health forced his retirement before he was promoted to four-star grade, which he had so clearly earned.

[20][26] After the war ended, Simpson commanded the Memphis, Tennessee-based Second United States Army from 11 October to 14 November 1945.

[26] On 4 August 1954, he was promoted to full general on the retired list by a special Act of Congress that advanced officers who had commanded armies or the equivalent to that rank.

He was also chairman of the board of the Alamo chapter of the Association of the United States Army, and spearheaded a drive to raise $750,000 for the construction of the Santa Rosa Children's Hospital.

[5][21] His wife Ruth died in 1971, and soon thereafter, Simpson moved into the Menger Hotel in downtown San Antonio, where he was very popular with the staff.

In 1978, at the age of 90, he met Catherine Louise (Kay) Berman, a retired civil-service worker from a military family 33 years his junior, and the two were married on 9 April 1978.

[21][61] Simpson died in the Brooke Army Medical Center on 15 August 1980,[22] and was buried alongside his first wife Ruth in Arlington National Cemetery in Virginia.