Davis, who headed the federal commission charged with giving meaning to the holiday in 1983, died in his home Monday after a battle with colon cancer, the Los Angeles Times reported Thursday.
"He was a person who worked untiringly to bring about the holiday," said King's sister, Christine King Farris. "He knew the inner workings of the government, and because of that background he was able to push forward."
Davis, a longtime employee at the U.S. Department of Housing and Urban Development, worked behind the scenes with King's widow, Coretta Scott King, to get the holiday made official.
"He just wanted to orchestrate and be in the background," said Davis' stepdaughter, Tracy Reid, of San Anselmo, Calif. "He wouldn't flaunt his own accomplishments."
In addition to his stepdaughter, Davis is survived by a daughter, Leigh Susan Davis, of Galway, Ireland; and a stepson, Steven Kirk, of Chevy Chase, Md.
Memorial donations can be sent to the King Center, 449 Auburn Ave. NE, Atlanta, GA 30312.
Jordana Brewster on Paul Walker: 'He was an enormous presence in my life'
Gal Gadot cast as Wonder Woman for 'Batman vs. Superman'