Clifford L. Alexander Jr., whose long career as a leading adviser to Democratic presidents ranged from working behind the scenes on landmark legislation like the Voting Rights Act to high-profile roles like serving as the first Black secretary of the Army, died on Sunday at his home in Manhattan. He was 88.

His daughter, the poet Elizabeth Alexander, said the cause was heart failure.

Mr. Alexander was a lifelong devotee of the promises held out by President Lyndon B. Johnson and his Great Society, in particular the idea that government could do much to alleviate racial and economic inequality. And he was among the generation of young Black leaders who, in the 1960s and ’70s, brought the civil rights movement from the streets into the machinery of the federal government.

As chairman of the Equal Employment Opportunity Commission under President Johnson and, briefly, his successor, Richard M. Nixon, Mr. Alexander turned what had been a relatively powerless agency into a central player in fighting workplace discrimination. He resigned after Nixon demoted him from chairman to commissioner, criticizing the president for “a crippling lack of administration support.”

Later, as the secretary of the Army under Jimmy Carter, he opened the doors for Black officers to rise to the rank of general, including a particularly promising young colonel named Colin Powell.

“Cliff saw his role as secretary of the Army as a key extension of the civil rights movement, and he inaugurated and enforced policies that were spectacularly effective in achieving his goal,” the Harvard professor Henry Louis Gates Jr., a longtime friend, said in a phone interview. “The fact that the United States military is, perhaps, the most integrated institution in our society can be traced to the foresight of Clifford Alexander.”

Mr. Alexander was among the few Black leaders to be openly critical of President Bill Clinton, arguing that he engaged with race superficially and only when it was politically expedient. But he was a major supporter of Barack Obama, both as an adviser and as a campaign surrogate during Mr. Obama’s run for the White House in 2008.

Coincidentally, his daughter, who was then a professor of poetry at Yale and a longtime friend of the Obamas, read her poem “Praise Song for the Day” at Mr. Obama’s inauguration in 2009.

“Cliff was an American original — a civil rights trailblazer whose eyes were never shut to injustice but whose heart was always open,” Michelle Obama said in a statement. “He was like a father to me and an inspiration to Barack. We admired the way he fought and learned from the way he led.”

Clifford Leopold Alexander Jr. was born on Sept. 21, 1933, in Harlem. His father was a Jamaican immigrant who managed the Riverton Houses, a sprawling residential development in Harlem financed by the Metropolitan Life Insurance Company. Unlike other Met Life developments, including Stuyvesant Town-Peter Cooper Village in Manhattan, Riverton was integrated, and most of its residents were Black.

Mr. Alexander’s mother, Edith (McAllister) Alexander, was also active in the city’s life and politics. She served several mayors as an adviser on civil rights. She is believed to have been the first Black female elector at a Democratic National Convention, in 1948.

After attending the Fieldston School, a private high school in the Bronx, Mr. Alexander studied government at Harvard, where he was elected the first Black president of the student council. He graduated in 1955 and received his law degree from Yale in 1958.

Back in New York, he worked for a time as an assistant district attorney and as the executive director of Harlem Youth Opportunities Unlimited, an antipoverty organization founded by Kenneth Clark and Mamie Phipps Clark.

He married Adele Logan, a historian, in 1959. Along with their daughter, she survives him, as do their son, Mark, and seven grandchildren.

Both of Mr. Alexander’s children went on to successful careers: Elizabeth is now the president of the Mellon Foundation, and Mark is the dean of the Charles Widger School of Law at Villanova University.

Mr. Alexander arrived in Washington in 1963 to serve on the staff of the National Security Council under President John F. Kennedy. Almost immediately, he was also acting as an informal adviser on race, and Kennedy sent him as an observer to the March on Washington.

‘’The White House was in a state of clear apprehension,” Mr. Alexander told The New York Times in 2003. “If you get in a position like the one I was in, you have a responsibility to say to the people in power what you think about race. So I went out to see what was happening.”

Not long after Kennedy’s assassination, Johnson brought Mr. Alexander into his circle to act as a liaison to the civil rights movement and, in particular, the Rev. Dr. Martin Luther King Jr.

Mr. Alexander soon became Johnson’s closest adviser on race relations, entrusted with lining up support in the Black community for the president’s legislative priorities and helping shepherd Black nominees through Congress, including Robert C. Weaver as the secretary of housing and urban development and Thurgood Marshall as a Supreme Court justice.

Even after Johnson appointed Mr. Alexander chairman of the Equal Employment Opportunity Commission in 1967, he continued to rely on him as a conduit to the Black community. When Dr. King was assassinated and violence erupted across Washington, Johnson sent Mr. Alexander into the streets to meet with Black leaders and to assess the damage.

After leaving the Equal Employment Opportunity Commission, Mr. Alexander became the first Black person to achieve the rank of partner at a major Washington law firm when he joined Arnold & Porter. He hosted a syndicated TV talk show, “Black on White,” from 1972 to 1976, and ran unsuccessfully for mayor of Washington in 1974.

As secretary of the Army, he was charged with continuing to rebuild the armed forces after the disaster of the Vietnam War. It was a herculean task that involved reorienting the Army around volunteers, ending racial discrimination and bringing in more women.

His time in that post, which ended in 1981, was his last official stint in government service. But he continued to serve as an informal adviser to politicians and policymakers. He served on several corporate boards and, in the late 1990s, as the interim chairman and chief executive of the advisory firm Dun & Bradstreet.

He and his wife founded a consulting firm, Alexander & Associates, that advised major corporations on how to reduce racial inequality. Among their most notable clients was Major League Baseball, which they helped address racial disparities in the organization’s front offices.

Among his tidbits of advice was the following, on the importance of getting people to pay attention to you.

“Very few senators or members of Congress do things just because it’s right, or we’d have a far better world than we have today,” he said in a 2017 interview for the Kunhardt Film Foundation. But, he added, “If you can show somebody why it is in their interest, they may do some things.”