The name "Duke" is not a title, but a given name. His father was named "Duke" in honor of Prince Alfred, Duke of Edinburgh, who was visiting Hawaii at the time of the elder man's birth in 1869. The younger "Duke", as eldest son, inherited the name.
In his youth, Kahanamoku preferred an old-school board, which he called his "papa nui", constructed after the fashion of ancient Hawaiian "olo" boards. Made from the wood of a koa tree, it was sixteen feet (4.8 m) long and weight 114 pounds (52 kg). The board was without a skeg, which had yet to be invented. In his later career, he would often use smaller boards, but always preferred those made of wood.
On August 11, 1911, in an amateur swim meet, Kahanamoku was clocked at 55.4 seconds in the 100-meter freestyle, beating the existing world record by 4.6 seconds, in the salt water of Honolulu Harbor. He also broke the record in the 220 and equaled it in the 50, but the Amateur Athletic Union, in disbelief, would not recognize these feats until many years later.
Nevertheless, Kahanamoku easily qualified for the U.S. Olympic swimming team in 1912, breaking the record for the 200 meter freestyle in his trial heat for the 4×200 relay. He went on to win a gold medal in the 100 yard freestyle in Stockholm, and a silver with the relay team. In Antwerp in 1920, he won gold medals both in the 100 (bettering fellow Hawaiian Pua Kealoha) and in the relay. He finished the 100 with a silver medal in Paris in 1924, the gold going to Johnny Weissmuller and the bronze to Duke's brother, Samuel Kahanamoku.
Between Olympic competitions, and after retiring from the Olympics, Kahanamoku traveled to different places in the world, particularly Australia and the United States, to give swimming exhibitions. It was during this period that he popularized the sport of surfing, previously known only in Hawaii, by incorporating surfing exhibitions into these visits as well.
During his time in southern California, Kahanamoku also performed in Hollywood as an extra and a character actor in several films. In this way, he made connections with people who could further publicity for the sport of surfing.
Duke Kahanamoku was the first person to be inducted into both the Swimming Hall of Fame and the Surfing Hall of Fame. The Duke Kahanamoku Invitational Surfing Championships are named in his honor. He served as sheriff of Honolulu, Hawaii from 1932 to 1961.