Jim Nicholson was once presented with one of life’s great open questions: How would you describe yourself?

For some people, that could be an invitation for ego-stroking, a chance to paint the most flattering, fascinating portrait their imaginations can conjure.

Mr. Nicholson answered with the verbal equivalent of a shrug. He was a simple man, he said, not terribly complicated. Nothing much to see here, in other words.

You could fill a phone book with all of the things he left out. Mr. Nicholson was an accomplished investigative journalist and a legendary obituary writer at the Philadelphia Daily News, where he spent 19 years celebrating the lives of men and women who were the true lifeblood of the city — the cooks, carpenters, bus drivers and janitors whose stories often went untold.

His approach to obituary writing won him national acclaim and a legion of imitators, but the newspaper was just his day job. Mr. Nicholson was also a bronze star-winning counterintelligence officer in the Army, who often disappeared for months at a time on top-secret missions. He briefly came out of retirement in 2008, at the request of then-Gen. David Petraeus, to help track insurgents in Iraq.

Mr. Nicholson, of Gloucester City, died from heart failure on Friday at Cooper University Hospital in Camden. He was 76.

“He lived at least four different lifetimes,” his son Jim said on Saturday. “But this is a man who was so much more than his accomplishments. And that’s saying a lot, considering how many accomplishments he actually had.”

Mr. Nicholson was born in Philadelphia in 1942. He graduated from Gloucester City High School in 1960, became a reservist in the Marine Corps, and then obtained a bachelor’s degree in journalism from the University of Southern Mississippi.

He tried on careers like disguises, with a resumé that included dock worker, car salesman, private detective, and police intelligence analyst. His newspaper career, too, featured stints with a number of outlets, including the Wilmington News-Journal, the Courier-Post and the Inquirer.

Journalism proved to be a fine use of Mr. Nicholson’s innate ability to put people at ease and convince them to cough up information. Former Daily News reporter Kitty Caparella recalled being awestruck by the investigative reporting Mr. Nicholson did in the 1970s on the Black Mafia for Philadelphia Magazine, and outlaw motorcycle gangs for the Philadelphia Bulletin. “The cops didn’t even believe there was such a thing as the Black Mafia until he documented it,” she said. “He was an icon.”

Mr. Nicholson began working at the Daily News in 1978. In a newsroom filled with eccentric characters, he still managed to stand out, perpetually dressed, as he was, in dark suits with skinny ties and a face that could be described as a poor man’s Clark Gable.

But his career stalled after he had a falling out with the paper’s editor, Gil Spencer. In 1982, then-managing editor Tom Livingston asked Mr. Nicholson if he would consider writing obituaries. To nearly everyone’s surprise, he agreed.

He decamped to a mostly deserted corner on the 14th floor of the paper’s old headquarters at Broad and Callowhill Streets — away from meddling editors, he later explained — and treated everyday Philadelphians’ obituaries with the sort of care that was normally reserved for celebrities and heads of state.

Their ranks included Tony, a Kensington barber who tried to solve his customers’ legal problems from his barber’s chair; Aunt Dot, a North Philly church worker who was adept at corralling the neighborhood drunks; and Pop-Pop, a dapper Northeast Philly ward leader who hid his feet behind a chair if visitors happened to come over while he was wearing slippers.

“He made them just as important as anyone else, and showed that everyone had a story to tell,” Livingston said.

Mr. Nicholson said he learned how to write obituaries from reading the work of his counterpart at the Inquirer, Burr Van Atta, and talking to him frequently, sharing two or three coffee breaks a day. “He was my hero,” Mr. Nicholson said upon Van Atta’s death in 1993.

Andrea Mihalik portrait of Jim Nicholson, Philadelphia Daily News, June 17, 1991
ANDREA MIHALIK / Staff Photographer
Andrea Mihalik portrait of Jim Nicholson, Philadelphia Daily News, June 17, 1991

Mr. Nicholson’s interest in the private lives of homemakers and bartenders wasn’t limited to his working hours. He took an avid interest in nearly everyone with whom he crossed paths. Bank tellers and pharmacists could expect to receive poinsettia plants from Mr. Nicholson at Christmas; trash collectors knew he’d have bottles of Coca-Cola waiting for them in the summer.

“He truly felt like he was the Everyman, and he looked out for everybody,” Jim said of his father. “He believed in treating everyone with dignity and respect.”

Mr. Nicholson joined New Jersey’s National Guard in 1982, and later transferred to the Army Reserve, rising to the rank of lieutenant colonel. It was about that time that he began leaving the Daily News for overseas military missions that he rarely discussed.

Former Daily News editor Zachary Stalberg remembered that Mr. Nicholson was once annoyed with him over some work issue before he departed for several months. “And then I was getting on the elevator one day, and there was Nicholson with a 3½-foot machete in his hand,” Stalberg said. “And I figured, ‘Nicholson has reappeared to behead me!’ He chased me down and said, ‘No, no, no, this is a present for you.’”

Mr. Nicholson was sent on nearly a half-dozen missions to Panama in the late 1980s and early ‘90s, and was intimately involved in the 1989 U.S. invasion that overthrew Gen. Manuel Noriega.

“I can’t think of anybody else who was so devoted to two separate careers,” said former Daily News staff writer John F. Morrison, who inherited the obit beat after Mr. Nicholson retired. “He was just amazing.”

In 1992, Mr. Nicholson was working on a counter-operation in Panama regarding drugs when he met Army Lt. Col. Harris Arlinsky. A number of high-ranking military officials were involved in the operation, but Arlinsky was stunned to find that they were taking orders from Mr. Nicholson, who was below them on the chain of command.

“He was not the senior man, but he was the man in charge,” explained Arlinsky, now retired. “It’s close to impossible to have something like that happen. Usually somebody’s going to say, ‘Don’t tell me what to do, I outrank you.’ But Jim just had a presence in the way that he conducted himself.”

Other intelligence missions centered on Colombia, Venezuela and the U.S. Border Patrol. Somewhere along the way, Mr. Nicholson struck up a correspondence with Petraeus, and the two men became friendly.

“Jim Nicholson was a very thoughtful adviser and one of those who always sought to ‘speak truth to power,' as the saying goes,” Petraeus wrote in an email on Sunday. “He was one who repeatedly provided forthright and thoroughly researched assessments, and I valued his input very highly.”

Retired Navy Capt. Bill Battle, a longtime friend of Mr. Nicholson’s, was serving in the Iraq War when he discovered that Mr. Nicholson was still in contact with Petraeus. The general soon extended an offer: Would Mr. Nicholson consider coming out of retirement, at age 66? He couldn’t say no.

Within a few months, he was virtually running an intelligence department in Baghdad. “The guy was just a ruthless worker,” Battle said. “Even those of us who felt like we had a strong work ethic almost felt inadequate around him.”

In 2012, Mr. Nicholson published a 783-page memoir on military analysis, Because No One Else Can: Inside the Military Intelligence Secret Sausage Factory.

The title came from a Batman movie in which, after dispatching several thugs, the caped crusader is asked by a little girl, “Why do you do this?”

“Because no one else can,” Batman replies.

In the book, Mr. Nicholson examined how smarts, focus, and determination drive the best intelligence agents, influencing the way they build facts into deductions, and how those conclusions help shape United States policy.

“One of the greatest rewards an analyst can have is to know that he or she played a major role in actually directing an investigation, operation, mission or policy,” he wrote. “’Did my being here make a difference?’ … The analysts may never be able to answer that question. But, they do know why they do what they do. It is because no one else can.”

While the globe-trotting was fascinating and his writing was celebrated, neither defined Mr. Nicholson. To his family, he was a Sunday school teacher, a painter, and a kind and patient father to three boys: Jim, David and Jeffrey. To them, he stressed the importance of kindness and hard work.

“He used to quote his grandfather to us,” Jim said, “and say, ‘The greatest way to get rid of an enemy is to make him your friend.’”

Mr. Nicholson — who is also survived by a brother, Robert — had separated from his wife, Betty Jo Williams Pratt, years ago. But when she was diagnosed with Alzheimer’s in the late 1990s, he moved in with her and became her constant caregiver until she died in 2011.

“He never complained. It was not a burden for him,” Jim said. “When he looked back on it, he told me taking care of her was the best job he ever had.”

Mr. Nicholson was nominated for the Pulitzer Prize six times, and wrote two self-published textbooks on military intelligence. He received an added dose of recognition when author Marilyn Johnson highlighted his obituary writing in her 2006 book, The Dead Beat: Lost Souls, Lucky Stiffs, and the Perverse Pleasures of Obituaries.

It was Johnson, a former Esquire editor, who had asked Mr. Nicholson how he would describe himself. After decades of narrating the lives of others — their heartaches, triumphs and legacies — Johnson wanted Mr. Nicholson to tell his story.

“He stepped back,” she said. “He was completely baffled.”

It was the first and probably only time that Jim Nicholson was at a loss for words.

Mr. Nicholson will be buried at Arlington National Cemetery.

Staff writers Lisa Gartner and Jeff Gammage contributed to this article.