In a remarkable feat of theatrical longevity, actor Hal Holbrook, who died Jan. 23 at 95, played Samuel Clemens — better known as Mark Twain — in a solo show for nearly as long as the American humorist and iconoclast was alive.

“Mark Twain Tonight!” — which Mr. Holbrook conceived and debuted in 1954 — earned him a Tony Award on Broadway in 1966 and captivated more than 20 million viewers in a CBS telecast in 1967. Into his 90s, he was still crisscrossing the globe, weaving Twain’s stories and quips into a peppery monologue about mankind’s pretensions and vices.

As he explained in character, “Man is the only animal that blushes — or needs to.”

Twain, the sly, white-haired, white-suited storyteller remembered for such books as “The Adventures of Tom Sawyer” and “The Adventures of Huckleberry Finn,” died in 1910 at 74. Through his immersion in the role, which he performed more than 2,200 times, Mr. Holbrook allowed generations of theatergoers to meet Twain as the walrus-mustached author appeared in his final years, reciting his comic aphorisms and doing staged readings of his best-loved works.

“Holbrook hasn’t actually become Twain, but he has reached a level of such commanding spontaneity that it nearly amounts to the same thing,” Washington Post theater critic James Lardner once wrote. “It is the voice that carries the performance — a voice that sounds, simply, the way Twain’s prose reads. It sounds like the voice of a 70-year-old, cigar-smoking, Missouri-born genius who has been a lot of places and seen a lot of things.”

The play showcased the actor’s virtuosity, sliding between nuggets of wry misanthropy and great poignancy, channeling the old Twain channeling the young rascal Huck, who recounts his friendship with Jim, a man running away from slavery.

Mr. Holbook’s career spanned seven decades and 130 films and TV shows. Although playing Twain threatened to engulf the lot, Mr. Holbook maintained a hectic career as a Hollywood character actor known for delivering taut and intelligent performances.

In “All the President’s Men” (1976) he was Deep Throat, the shadowy unnamed source who advises Washington Post journalist Bob Woodward (played by Robert Redford) to “follow the money” on the trail of the Watergate scandal.

No one in the film’s cast and crew knew the real Deep Throat’s identity — although in selecting Mr. Holbrook, the director, Alan J. Pakula, said he sought assurance from Woodward that the whistleblower was not “Tricia Nixon or Golda Meir or someone who’s so far removed from my casting that it’s going to make the picture look ridiculous” in the future.

It fell to Mr. Holbrook to shade the character, a stern and conspiratorial enigma who favored clandestine meetings with Woodward in dark parking garages.

“The backstory I created for him was based on a question of morality,” Mr. Holbrook told the New York Times in 2005 after Deep Throat was revealed to be W. Mark Felt, the No. 2 official at the FBI. “This man was breaking the rules of Washington by telling on the president. I thought, ‘What kind of man might do that?’

“I figured he’d served in administrations of both parties — an adviser like Clark Clifford,” he said. “And now he was faced with the choice of loyalty to his president or loyalty to his country. He chose his country. But it was so distasteful to him because he was a person of character and here he was, passing information along in a garage.”

With his wavy hair, gravelly voice, sturdy jaw line and air of knowing authority, Mr. Holbrook masterfully portrayed a range of historical figures. In the 1970s, he won Emmy Awards for playing President Abraham Lincoln in one miniseries (based on Carl Sandburg’s Pulitzer Prize-winning biography) and Lloyd Bucher, the ill-fated Vietnam War-era Navy commander of the USS Pueblo, in another. In the Steven Spielberg film “Lincoln” (2012), he was the conservative gray eminence Preston Blair.

Mr. Holbrook received his first Emmy Award for his performance as a crusading liberal politician in the short-lived NBC drama series “The Bold Ones: The Senator” (1970). And he was a veteran stockbroker who counsels the brazenly ambitious Charlie Sheen in “Wall Street” (1987) — “a sort of Deep Throat with a day job,” as Post film critic Desson Howe described the part.

The actor also excelled as outwardly respectable lawmakers, judges and police officers who harbored secrets — from the duplicitous police lieutenant in the “Dirty Harry” sequel “Magnum Force” (1973) to the nefarious law firm partner in “The Firm” 1993), based on John Grisham’s thriller.

With Martin Sheen as his lover, Mr. Holbrook was deeply affecting as a gay man struggling to explain his uncloseted life to his son in “That Certain Summer” (1972), a groundbreaking network-television film (for ABC) that did not play homosexuality for crude laughs or melodrama.

Actor and filmmaker Sean Penn, who long admired Mr. Holbrook’s work, cast him in “Into the Wild” (2007) as a widowed loner who becomes briefly a surrogate father to a young adventurer headed to Alaska (Emile Hirsch). The movie, based on the nonfiction bestseller by Jon Krakauer, brought the 82-year-old Mr. Holbrook his only Academy Award nomination.

Mr. Holbrook once told an interviewer that he chose roles sometimes to get far, far away from Twain’s shadow. “I’ll never be able to satisfy myself,” he said, “until I prove myself without that wig on.”

An unlikely beginning

Harold Rowe Holbrook Jr. was born in Cleveland on Feb. 17, 1925. His father spent years shuttling between jail cells and mental institutions, and his mother, a onetime vaudeville dancer named Aileen Davenport, abandoned him as a baby.

He was brought up by his paternal grandparents in South Weymouth, Mass., and at 7 was sent to boarding school, graduating in 1942 from the Culver Military Academy in Indiana.

A lonely and lackluster student with little aptitude for athletics, he found sanctuary in a drama class and then a calling as an actor. In his 2011 memoir, “Harold: The Boy Who Became Mark Twain,” he wrote of seeking “to make myself disappear” in costumes and wigs.

While serving in the Army in Newfoundland during World War II, Mr. Holbrook joined an amateur theater club and married one of the actresses, Ruby Johnston. In 1948 they graduated from Denison University in Ohio, then spent five grueling years in a two-person road show of “Great Personalities” of history and literature that played at school assemblies and women’s clubs in 46 states. One of the characters was Twain.

Mr. Holbrook settled in New York in 1952 and found steady work on the daytime CBS soap opera “The Brighter Day” but had few other acting prospects. After reading about the global triumph of Welsh actor Emlyn Williams’s recitations of the work of Charles Dickens, Mr. Holbrook figured that he could do the same with a one-man Twain show.

He polished “Mark Twain Tonight!” for five years before it became a critical sensation off-Broadway in 1959. Later that year, the actor played at President Dwight D. Eisenhower’s birthday party. An 11-country tour followed, under the auspices of the State Department.

In the mid-1960s, Mr. Holbrook had a frustrating experience with the fledgling Repertory Theatre of Lincoln Center — winning meaty roles in works by Eugene O’Neill, Arthur Miller and Molière but soon finding himself among those “axed” after conflicts with new management. He was 41 when he made his movie debut, in a supporting role in “The Group” (1966), based on the Mary McCarthy novel about Vassar graduates.

Although film stardom proved elusive, Mr. Holbrook became an almost constant presence on television. He played Lincoln again in the mid-1980s for the ABC television miniseries “North and South” and its sequel. In 1989 he won an Emmy for narrating the documentary series “Portrait of America” for the cable channel TBS. He had cameo roles in many later shows, including “Evening Shade,” “The West Wing,” “Sons of Anarchy” and “The Sopranos.”

He continued to bring a welcome astringency to salt-of-the-earth roles in movies such as “That Evening Sun” (2009), which offered him a rare leading part, and in Gus Van Sant’s “The Promised Land” (2012), an anti-fracking drama starring Matt Damon.

His marriages to Johnston and actress Carol Rossen, daughter of filmmaker Robert Rossen, ended in divorce. His third wife, actress Dixie Carter, died in 2010.

Survivors include two children from his first marriage, Victoria Holbrook and David Holbrook; a daughter from his second marriage, Eve Holbrook; two stepdaughters, Ginna Carter and Mary Dixie Carter; and four grandchildren.

Mr. Holbrook died at his home in Beverly Hills, Calif., said his grandson Will Holbrook, who did not provide a specific cause.

In all his years of performing “Mark Twain Tonight!,” a 63-year run that ended in 2017, Hal Holbrook took greatest pride in his ability to relate Twain’s piquant lines to contemporary politics and race relations.

On a Southern tour in 1962, he performed at the University of Mississippi days after James Meredith’s enrollment as the first Black student provoked mob violence. Onstage as Twain, Mr. Holbrook talked about the silence maintained by most people amid great injustice. “There is no art to this silent lie,” he said. “It is timid and shabby.”

He let the words hang in the air.

“Only three times in 49 years has that moment ever gotten applause,” he told the Times in 2003. “The first time was in Hamburg, Germany, in 1961. The second time was that night in Oxford, Miss. The third time was in Prague, behind the Iron Curtain, in 1985.”

He added: “I’m certainly surprised that I can be so riveted on this guy, on what he’s talking about and thinking about after all these years. But I can’t wait to get on the stage with this guy. Because I’m going to be able to say something.”