McNamara was born on August 10, 1967 in Pittsfield, Massachusetts and spent part of his childhood in Berkeley, California.
In Berkeley, McNamara, and his younger brother Liam McNamara, were both known by friends to be rather fearless and to shrug off the pains of rough childhood play, foreshadowing his later ability to face danger while surfing. McNamara moved to the North Shore of Hawaii in 1978. At eleven years old, McNamara followed his younger brother's footsteps and began surfing at Sunset, Waimea and the outer reefs in search of giant swells. He entered and placed in the prestigious Hawaiian Triple Crown Series at seventeen and began to gain major sponsors from major Japanese brands. For the next ten years both brothers joined the competition circuit, traveling and becoming fluent in Japanese.
Tow surfing caught on amongst the surfing community in the early 1990s and McNamara was one of the first to join in on the rush. Boats and personal water craft enabled surfers to chase down and catch giant waves that were thought impossible, beyond the bounds of surfers paddling with their bare hands. McNamara welcomed and craved the challenge to find the biggest waves in the world, which would instantly become his dream and mission in life.