William Simpson was born May 18, 1888, at Weatherford, Texas.
After graduating from the United States Military Academy in 1909, he was commissioned into the infantry. Before U.S. involvement in World War I, Simpson served in the U.S. and in the Philippines, including the Pancho Villa Expedition, in 1916.
He was promoted to Captain in May 1917 and served with the 33rd Division throughout World War I, receiving temporary promotions to Major and Lieutenant Colonel and becoming divisional Chief-of-Staff.
In the inter-war years, 1919–1941, Simpson filled staff appointments and attended military schools, both as student and as instructor. From 1932 to 1936, he served as the Professor of Military Science at Pomona College in Claremont, California. From April to September 1941 he was the first commander of the country's largest Infantry Replacement Training Center, Camp Wolters, locate in Mineral Wells, Texas.
In mid-1940, he was appointed to command the Ninth Infantry at Fort Sam Houston, Texas. Before U.S. entry into World War II, he had commanded divisions and received a promotion to temporary Major-General, taking the 35th Division from Camp Robinson, Arkansas, to a training site in California.