He was one of TV’s most trusted sidekicks, first among the great detective’s able lieutenants. Every week, his boss gave the order that would cement them both into the annals of popular culture: "Book ’em, Danno."

Despite a career that spanned 40 years in film and television, history will always remember James MacArthur for his portrayal of Danny "Danno" Williams on the original version of CBS Television’s "Hawaii Five-0."

It was how MacArthur was remembered as family, friends and fans mourned the actor’s death yesterday. MacArthur died at the Mayo Clinic in Jacksonville, Fla. He was 72.

MacArthur’s family would not release a cause of death, saying only that he died of "internal complications." His publicist, Richard Lewis, later released a statement that he died of "natural causes."

He was surrounded by loved ones, said Charlie MacArthur, the oldest of the actor’s four children.

"He died peacefully and the whole family was there," he said. "It was nice to have everyone together. We were very fortunate."

The actor’s son said funeral plans were still being sorted out but would include memorial services in Hawaii and MacArthur’s longtime home of Palm Desert, Calif.

Earlier yesterday his wife, former golf pro Helen Beth "H.B." MacArthur, told Hawaii entertainer Jimmy Borges that her husband might be buried in New York beside his mother, actress Helen Hayes.

Besides his wife and son Charlie, MacArthur is survived by children James D. MacArthur, Mary McClure and Juliette Rappaport; and seven grandchildren. MacArthur’s first two marriages, to actresses Joyce Bulifant and Melody Patterson, ended in divorce.

MacArthur was a starring cast member of the series from 1968 to 1979, leaving one year before it ended.

Ironically, he was not the first choice when the "Five-0" pilot was shot. The part had been played in the pilot by Tim O’Kelly, but audiences didn’t warm to him. MacArthur’s tough yet sincere portrayal made his Danno one of television’s most enduring characters.

Some of the others in the ensemble died earlier: Jack Lord, who played Steve McGarrett, died in 1998; Kam Fong, who played Chin Ho Kelly, in 2002; and Gilbert Lani "Zulu" Kauhi, who played Kono Kalakaua, in 2004.

"This is the end," said Rose Freeman, widow of the show’s creator, Leonard Freeman, "but he will never be gone. He will always be on that screen, and we will always see him. We will see all of them."

CBS had hoped to find a role for MacArthur in its current reboot of "Five-0," and as recently as last month, the actor’s personal website said he had agreed to appear in an episode.

When CBS premiered the new series at Sunset on the Beach in Waikiki, friend and former "Five-0" regular Al Harrington read a statement from MacArthur to the audience, which cheered when he told them the actor hoped to have a guest appearance.

Yesterday, Harrington called MacArthur an actor who wanted to make "Five-0" as good as it could be.

"He wasn’t caught up in his fame," Harrington said in a statement. "He had a great sense of what the aloha spirit was all about, which a lot of people don’t understand even after being here so many years."

The new cast of "Five-0," which includes Scott Caan as Danno, learned of the actor’s death yesterday. Peter Lenkov, the new show’s executive producer and a big fan of the original series, said in a statement, "James was a great actor, admired by so many. Since the early development of our reboot, I had hoped to include James in the show, in a cameo or a recurring role. … Alas, that will never happen now, and today I join millions of fans mourning this great loss."

MacArthur was born Dec. 8, 1937, in Los Angeles. He was adopted as an infant by actress Helen Hayes and playwright Charles MacArthur, so it’s not surprising that he spent his life onstage and in front of the camera.

MacArthur never quite outgrew his boyish looks. It was this blend of wholesome, all-American looks and an edgy toughness that won him the role of Danno.

His first acting job was as a child, alongside his mother in a radio play performed before a live audience. As a teenager, MacArthur appeared in summer-stock productions, breaking through as a misunderstood teen in John Frankenheimer’s "Deal a Blow," on live television. It went over so well that Frankenheimer remade it as a feature film, "The Young Stranger," again starring MacArthur. Next came a role as a boy captured by Indians in the Disney film "The Light in the Forest," and MacArthur’s work was so intense that he won notice as a promising newcomer.

He worked steadily thereafter — as a young adventurer in "Kidnapped," "Third Man on the Mountain" and "Swiss Family Robinson"; as a psychopathic murderer in "The Untouchables"; as a naive man who must rely on inner strength in "The Interns," "The Bedford Incident" and "Spencer’s Mountain"; and as an Army lieutenant in "Battle of the Bulge."

After "Five-0," MacArthur appeared mostly onstage, and made TV appearances in programs like "The Love Boat," "Vega$," "Fantasy Island" and "Murder, She Wrote" as well as TV movies "The Night the Bridge Fell Down" and "Alcatraz: The Whole Shocking Story." His last TV role was in "Storm Chasers: Revenge of the Twister" in 1998.

Local TV producer Emme Tomimbang became a good friend of MacArthur after the two created a documentary in 1996, "Memories of ‘Hawaii Five-0.’" She would see her friend every summer, when MacArthur spent time at his Waikiki apartment, she said. She introduced him to her hairstylist, and the two often played golf.

MacArthur told Tomimbang stories about the practical jokes on the set of "Five-0" that left her sides hurting — like the time he locked the police car so that when it rolled to a stop in an action scene, Kam Fong couldn’t get out.

"Sometimes I would forget who he was," she said. "I would forget he was Danno, the guy on ‘Hawaii Five-0.’ He was someone I enjoyed hanging around with. We just laughed a lot."

That casual way with people carried over onto the screen. Jimmy Borges, who appeared in a variety of "Five-0" episodes, marveled at MacArthur’s skill.

"When he was on the set and doing a scene, you couldn’t catch him acting," Borges, now 75. "He just flowed within the scene."

MacArthur was a natural whose flawless and selfless delivery never failed to set the stage for his boss, Lord’s McGarrett. Like a straight man, it was the sidekick’s job.

"He set up Jack for his big lines," Borges said. "He gave Jack the opportunity to have the big moment. He was sacrificing his ego for the scene, for the art, and Jimmy did that very naturally and he was fine with that."