Ninja Master, Marital arts, instructor, author, Buddhist priest. Some of the many titles used to describe Stephen K. Hayes. In the 1970s, Hayes traveled to Japan In June 1975, he met and began training under Tsunehisa Tanemura and then after Tanemura left Masaaki Hatsumi , who refers to himself as 34th Grandmaster of Togakure-ryū ninjutsu and is the founder of the Bujinkan (武神館) organization.

Hayes returned to the United States with his wife, Rumiko, in late 1980 when his Japan residency visa expired, and began teaching and authoring numerous books and magazine articles.

In 1985, he was entered into the Black Belt magazine's Hall of Fame as Instructor of the Year. He was again featured on the cover of Black Belt in the March 2007 issue.  The magazine's opening editorial described him as "one of the 10 most influential living martial artists in the world". In 1993, Hayes was awarded the judan (tenth-degree black belt) in Togakure-ryū ninjutsu from Hatsumi.

In 1997, he founded the martial art of To-Shin Do,  an art based in his experience of budō taijutsu.

In 2006, Hayes's name was removed from the display of active Bujinkan teachers, ending his official role within that organization. That his name plank was removed from the Bujinkan Honbu wall has been documented by both parties. However, the specifics regarding the motivation for the removal are in dispute. Some suggest Hayes was expelled from the organization. Hayes explains that he simply no longer participates as an active Bujinkan instructor. To-Shin Do is taught through his SKH (Stephen K. Hayes) Quest locations. The SKH Quest network now spans 31 schools across North America, Europe, Australia and Africa.

Hayes acted in the 1980 television miniseries Shogun and in 2004 was featured in the Discovery Channel documentary Unsolved History regarding ninja and their practices.