The name "Duke" is not a title, but a given name. He was named after his father, Halapu Kahanamoku, who was christened "Duke" by Bernice Pauahi Bishop 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.
Growing up on the outskirts of Waikiki (near the present site of the Hilton Hawaiian Village), Kahanamoku spent his youth as a bronzed beach boy. It was at Waikiki Beach where he developed his surfing and swimming skills.
In his youth, Kahanamoku preferred an old-school (traditional) surf 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 weighed 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 timed at 55.4 seconds in the 100 yard (91 m) 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 yd (201 m) and equaled it in the 50 yd (46 m), but the Amateur Athletic Union, in disbelief, would not recognize these feats until many years later. They initially claimed that the judges must have been using alarm clocks rather than stopwatches, and later claimed that ocean currents aided Kahanamoku.
Between Olympic competitions, and after retiring from the Olympics, Kahanamoku traveled internationally, 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. His surfing exhibition at Sydney's Freshwater Beach on December 23, 1914 is widely regarded as the most significant day in the development of surfing in Australia. The board Kahanamoku used is retained by the Freshwater Surf Club and can be viewed if the caretaker is approached respectfully. There is a statue of Kahanamoku on the headland at Freshwater. He also made surfing popular in mainland America first in Santa Cruz, California. This is where surfing first started in California.
During his time living 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. Kahanamoku was also involved with the Los Angeles Athletic Club, acting as lifeguard and competing on both swimming and water polo teams.
While living in Newport Beach, California on June 14, 1925, Kahanamoku rescued eight men from a fishing vessel that capsized in heavy surf while attempting to enter the city's harbor. Twenty-nine fishermen went into the water and seventeen perished. Using his surfboard, he was able to make quick trips back and forth to shore to increase the number of sailors rescued. Two other surfers saved four more fishermen. Newport's police chief at the time called Duke's efforts "the most superhuman surfboard rescue act the world has ever seen." Thus was born the tradition of lifeguards having rescue surfboards at the ready.
In 1940, he married the love of his life, Nadine Alexander. She accompanied him when he traveled all over the world.
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 is a member of the U.S. Olympic Hall of Fame.
He served as sheriff of Honolulu, Hawaii from 1932 to 1961, serving 13 consecutive terms.
Hawaii music promoter Kimo Wilder McVay capitalized on Duke's popularity by naming his Waikiki showroom Duke Kahanamoku's, and giving Duke a piece of the financial action in exchange for the use of his name. Duke's was a major Waikiki showroom in the 1960s and is best remembered as the home of Don Ho & The Aliis from 1964 through 1969.
Duke's name is currently being used by another business establishment, Duke's Canoe Club & Barefoot Bar, a beachfront bar and restaurant in the Outrigger Waikiki On The Beach Hotel. The decor includes many poster-size photos of Duke — with his brothers in Waikiki, and on his travels in other parts of the world. There is a chain of restaurants named after him in California and Hawaii called Duke's.
In 2002, Kahanamoku was featured on a first class letter rate postage stamp of the United States Postal Service.