Lenny Trujillo made a startling discovery when he began researching his descent from one of New Mexico’s pioneering Hispanic families: One of his ancestors was a slave.

“I didn’t know about New Mexico’s slave trade, so I was just stunned,” said Mr. Trujillo, 66, a retired postal worker who lives in Los Angeles. “Then I discovered how slavery was a defining feature of my family’s history.”

Mr. Trujillo is one of many Latinos who are finding ancestral connections to a flourishing slave trade on the blood-soaked frontier now known as the American Southwest. Their captive forebears were Native Americans — slaves frequently known as Genízaros (pronounced heh-NEE-sah-ros) who were sold to Hispanic families when the region was under Spanish control from the 16th to 19th centuries. Many Indian slaves remained in bondage when Mexico and later the United States governed New Mexico.