James Buchanan has been the only bachelor to serve as the President of the United States
More Info: His niece, Harriet Lane, served as his first lady, the first First Lady to not be married to the president. Buchanan's fiance broke off the engagement and then died of an opium overdose. His first Vice President, William King, was sometimes jokingly referred to as "Buchanan's wife". He and Buchanan had been roommates for 15 years. .
Grover Cleveland was also a bachelor when he was first elected president. In the first year of his presidency he married a 21-year-old college student, Frances Folsom, who was a daughter of one of his friends.