William DeWitt Mitchell, son of William, a lawyer who eventually became a state supreme court justice, and Frances Merritt Mitchell, was born in Winona, Minnesota on September 9, 1874. Mitchell received his early schooling in Winona Schools and the Lawrenceville School of New Jersey before enrolling in the Sheffield Scientific School at Yale University in 1891. Though he originally entered Yale with an interest in electrical engineering, after two years his focus shifted to law and he subsequently transferred to the University of Minnesota. To make up for lost time, Mitchell simultaneously took law courses at night while finishing his undergraduate degree. In 1895, he earned his bachelorís degree, followed by his law degree and admission to the bar in 1896. He later went on to be awarded honorary law degrees at Yale, Williams College, and the University of Michigan.
Mitchell practiced law in St. Paul, Minnesota in several firms. In 1902, he joined with two other prominent lawyers to form How, Taylor & Mitchell. They soon became one of the largest and most prosperous law firms in the upper Midwest. In 1919, Mitchell served on the regional council of the U.S. Railroad Administration and in 1922, he served as chairman of the Citizens Charter Committee of St. Paul. Mitchell served his country in both the Spanish-American and first World War. He served as a second lieutenant in the 15th Minnesota Volunteer Infantry and as acting judge advocate of the second Army Corps in the Spanish-American War. During his service in World War I he earned the rank of colonel.
Mitchellís appointment to solicitor general by President Coolidge on June 5, 1925 was his first public office. Though he went on to serve as U.S. Attorney General and chief counsel of the joint congressional committee investigating the attack on Pearl Harbor, Mitchell found solicitor general to be his most interesting experience. He held the position until March 4, 1929, at which time he was appointed Attorney General under President Hoover. In 1933, Mitchell moved to New York and joined a Wall Street law firm. He was named chairman of the Committee on Federal Rules of Civil Procedure, which were adopted in 1938 and were a major reform in the administration of justice.
William D. Mitchell passed away on August 24, 1955 in Syosset, NY. He left behind his wife of 54 years, Gertrude Bancroft, and a son, William Bancroft Mitchell.