When Native American activists from around the United States took over Alcatraz in 1969, George P. Horse Capture was a steel inspector for the California Department of Water Resources — a young man on his way to a solid career and ever further away from any sense of pride in his Montana reservation roots.

“I was very happy climbing that white mountain of success,” he once said. “But then I looked down over the top, and there was nothing there.”

The solution was to switch mountains. Joining the protesters for short periods over their 19-month stay, Mr. Horse Capture went on to become a passionate advocate for Native American culture and a museum curator who helped give his people an unprecedented voice in how their heritage would be presented and their artifacts displayed.

“He was profoundly important in contemporary American Indian history,” said Herman Viola, a longtime friend and curator emeritus of the Smithsonian’s National Museum of the American Indian in Washington....