Final answer:
William Howard Taft is the only person in U.S. history to have served as both President and Chief Justice of the Supreme Court.
Step-by-step explanation:
The only person in history who served as both the President of the United States and then as the Chief Justice of the Supreme Court was William Howard Taft. Taft was elected as the 27th President of the United States and served from 1909 to 1913. After his presidency, Taft achieved his lifelong ambition to become the Chief Justice of the United States, an office he held from 1921 to 1930. Often overlooked in discussions of the Supreme Court because of John Marshall's significant contributions as the Chief Justice from 1801 to 1835, which included the hallmark decision in Marbury v. Madison, Taft's career is unique for this rare combination of highest offices in the executive and judicial branches of the U.S. government.