Civil rights activist Al Sharpton released a statement confirming the death. The cause was not disclosed.
The Rev. Walker first met King in 1952, when both were young ministers, and he later became executive director of King's Southern Christian Leadership Conference. "I was Martin's alter ego," he said in 2012.
As pastor of Gillfield Baptist Church in Petersburg, Va., throughout the 1950s, the Rev. Walker became one of Virginia's most prominent voices for civil rights. He led the local chapter of the NAACP, was statewide leader of the Congress of Racial Equality and organized boycotts and sit-ins that often landed him in jail.
In one of his more public acts of resistance toward segregation, the Rev. Walker entered Petersburg's public library in 1960 and asked to check out a biography of Confederate Gen. Robert E. Lee.
"I was rubbing it in their noses a little," he told The Washington Post in 2016, "because I always felt Robert E. Lee was guilty of treason and should have been arrested and put in prison, but the South made such a hero of him."
The Rev. Walker was greeted with racial epithets. Along with several other African American protesters, he sat down on the library floor. "They called the police and arrested those of us who would not leave," he recalled. "They took us to the Petersburg jail, and that stirred up the community."
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He was fined $100 and sentenced to 30 days for organizing the demonstration.
"Petersburg was the center of the civil rights movement in Virginia," the Rev. Walker later told the Petersburg Progress-Index, and King took notice. In 1960, he asked the Rev. Walker to move to Atlanta to direct the SCLC, an organization that supported nonviolent protests and other civil rights protests.
"I told him, 'Whatever you want me to do, I'll do,' " the Rev. Walker recounted to the Progress-Index.
The Rev. Walker proved to be an adept fundraiser and organizer, quickly turning the SCLC into a regional force with a $1 million budget, more than 100 employees and chapters throughout the South. In the early 1960s, he helped organize the Freedom Rides, an effort to end segregation on interstate bus travel in the Deep South.
During one of the first forays of the Freedom Riders, in 1961, the Rev. Walker and his wife were among dozens of participants jailed in Jackson, Miss.
"We weren't there five minutes before we were arrested," he told The Post. They were held for more than a week.
In 1963, the Rev. Walker took a leading role in coordinating protests in Birmingham, Ala., a bastion of segregation, where King was arrested on April 12, 1963. During his eight days in the city jail, he wrote a powerful manifesto of the civil rights movement, smuggled out on scraps of paper to Rev. Walker. King's handwritten reflections were published as "Letter From Birmingham Jail," a seminal document of the era.
"Injustice anywhere is a threat to justice everywhere," King wrote. "We know through painful experience that freedom is never voluntarily given by the oppressor; it must be demanded by the oppressed."
Later, the Rev. Walker was a key planner of the Aug. 28, 1963, March on Washington, where King delivered his "I Have a Dream" speech. It was seen as a turning point, but within weeks, violence erupted in Birmingham, with the bombing of the 16th Street Baptist Church, killing four girls attending Sunday school.
The Rev. Walker redoubled efforts to organize a nationwide campaign of civil disobedience.
"We have been duped — or have duped ourselves — into believing the chains have been broken, when in truth we have only been chained more securely," he said. "Half-freedom has in many instances been worse than no freedom at all."
In 1964, the Rev. Walker left his leadership position at the SCLC and moved to New York, where he ran a publishing business specializing in books for African Americans and became an assistant pastor at the Abyssinian Baptist Church. In 1967, he took over the pulpit of the Canaan Baptist Church of Christ in Harlem, which became his pastoral home until his retirement in 2004.
He also served as a special assistant on urban affairs to Gov. Nelson A. Rockefeller (R-N.Y.) and advocated for affordable housing and better schools. In addition, he took a bullhorn to the streets to denounce drug dealers on the streets of Harlem.
"They were always warning me that it was dangerous, but that didn't stop me," he told the New York Times. "I had been involved in the struggle in the Deep South, so I was accustomed to dangerous situations."
The Rev. Walker spoke about spiritual matters and civil rights at gatherings throughout the world. He was an election supervisor in South Africa in 1994, when Nelson Mandela became the country's first post-apartheid president.
Wyatt Tee Walker was born Aug. 16, 1928, in Brockton, Mass. (His family said most biographical information about him incorrectly gives his year of birth as 1929.) He grew up in Merchantville, N.J., where his father was a minister.
The Rev. Walker graduated in 1950 from Virginia Union University in Richmond, where he studied chemistry and physics, and also received a master of divinity degree in 1953. He completed a doctorate from Colgate Rochester Crozer Divinity School in Rochester, N.Y., in 1975.
In addition to his ministry and civil rights work, the Rev. Walker was an authority on black religious music and wrote several books on the subject, including "Somebody's Calling My Name" (1979), "The Soul of Black Worship" (1984) and the three-volume "Spirits That Dwell in Deep Woods."
Survivors include his wife of 67 years, the former Theresa Ann Edwards of Chester; four children; a sister; and two granddaughters.
In 1967, the Rev. Walker returned with King to a demonstration in Birmingham, where they were both arrested. They shared a jail cell for five days.
The Rev. Walker secretly took a small camera into jail, taped to his leg. He took a photograph of a pensive King, peering through the bars of their cell, that became one of the most memorable images of the civil rights leader.
The next year, they were planning to be reunited in Petersburg for a march leading to Washington. It was scheduled to begin on April 4, 1968 — the day King was felled by an assassin in Memphis.
A previous version of this story incorrectly stated that Rev. Wyatt Tee Walker personally typed Martin Luther King Jr.'s manuscript of "Letter From Birmingham Jail." It was typed by Rev. Walker's secretary, Willie Mackey King.
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