Ernest Joseph King was born in Lorain, Ohio, on 23 November 1878, the second child of James Clydesdale King, a Scottish immigrant from Bridge of Weir, Renfrewshire, and his wife Elizabeth (Bessie) née Keam, an immigrant from Plymouth, England. His father initially worked as a bridge builder, but moved to Lorain, where he worked in a railway repair shop. He had an older brother who died in infancy, two younger brothers and two younger sisters: Maude (who died aged seven), Mildred, Norman and Percy.

The family moved to Uhrichsville, Ohio, when his father took a position with the Pennsylvania Railroad workshops, but returned to Lorain a year later. When King was eleven years old, the family moved to Cleveland, where his father was a foreman at the Valley Railway workshops, and King was educated at the Fowler School. He decided to go to work rather than high school, and took a position with a company that made typesetting machines. When it closed he went to work for his father. After a year, the family returned to Lorain, and King entered Lorain High School. He graduated as valedictorian in the Class of 1897; his commencement speech was titled "Uses of Adversity". The school was a small one; there were only thirteen classmates in his year.

King secured an appointment to the United States Naval Academy in Annapolis, Maryland, from his local Congressman, Winfield Scott Kerr, after passing physical and written examinations in Mansfield, Ohio, ahead of thirty other applicants. He entered Annapolis as a naval cadet on 18 August 1897. He acquired the nickname "Rey", the Spanish word for "king".

During the summer breaks, naval cadets served on ships to accustom them to life at sea. While still at the Naval Academy, King served on the cruiser USS San Francisco during the Spanish–American War. During his senior year at the academy, he attained the rank of cadet lieutenant commander, the highest naval cadet ranking at that time. He graduated in June 1901, ranked fourth in his class of sixty-seven and he was elected to serve at the head of the brigade. In thinking about American maritime policy, King often recalled the influence of the graduation address as given by the Vice President of the United States, Theodore Roosevelt, who handed out the diplomas