Martin was found dead Sunday by a roommate in his Van Nuys apartment, his longtime friend Lisa Lenes said.
Lenes said Martin had health problems in recent years and she believed he died of natural causes.
Martin, along with Young, Stills, singer-songwriter-guitarist Richie Furay and bassist Bruce Palmer, formed the band in 1966 and toured with the Byrds. Their debut album included the hit song “For What It’s Worth,” a solemn observation of 1960s turmoil.
They broke up two years later amid tension between Young and Stills. Young led a successful solo career and Stills moved on to Crosby, Stills & Nash. Furay formed Poco, another early country rock band.
Martin continued performing under various incarnations of the band. He and Palmer toured as Buffalo Springfield Revisited in the mid-1980s, and for a time in the 1990s he played shows as Buffalo Springfield Again.
Martin, also formed other groups, including Medicine Ball, which released one album.
Lenes said Martin, who was born in Chesterfield, Canada, will be buried in his native country at his family’s request. Information on his survivors were not immediately available.