Denny O’Neil, a leading comic-book writer who in the 1970s acquainted readers with Batman’s tougher, urban roots and injected social issues into the joint adventures of Green Lantern and Green Arrow, died on June 11 at his home in Nyack, N.Y. He was 81.

His son, Lawrence, said the cause was cardiopulmonary arrest.

Mr. O’Neil not only reinvigorated Batman and unified Green Lantern and Green Arrow; he also wrote comic books featuring Spider-Man, Iron Man, Doctor Strange, Daredevil and the Question in a 35-year career that included two stints at both DC Comics and Marvel Comics.

Jim Lee, DC’s chief creative officer and publisher, wrote on Twitter that Mr. O’Neil’s “focus on social issues pushed comics to wider respectability & acceptance as an art form,” adding, “Through his work & mentorship, he influenced generations of writers & artists.”

In 1970, Mr. O’Neil and the artist Neal Adams created a series in which Green Lantern and Green Arrow traveled the United States in stories that took on issues like racism, drug addiction and the environment.

In what has been called one of the most reprinted panels in comic-book history, an elderly black man confronts Green Lantern, in Green Lantern (Vol. 2, No. 76).

“I been readin’ about you,” he says. “How you work for the blue skins … and how on a planet someplace you helped out the orange skins … and you done considerable for the purple skins! Only there’s skins you never bothered with — the black skins. I want to know … how come?! Answer me that, Mr. Green Lantern!”

Without an excuse, Green Lantern says, “I … can’t.”

That was a big moment, according to Abraham Riesman, author of a coming biography of Stan Lee, Marvel’s patriarch. When he examined the issue for the Vulture website in 2018, Mr. Reisman wrote, “No one had played with the dynamite sticks of black dissatisfaction and white guilt like this in the genre before.” He called the scene “the moment superheroes got woke.”

Batman was another story. Mr. O’Neil and Mr. Adams (as well as other artists) rescued the Caped Crusader in the 1970s from the campy humor of the ’60s television series, which had infused DC’s storytelling. They recast Batman in the darker image his creators forged in 1939.

“As with Superman, there’s a tragedy at the center of his character,” Mr. O’Neil told the website Flickering Myth last year. “He is human, and that’s what attracted me as a writer.”

Using the social activist Dorothy Day as a model, Mr. O’Neil developed Leslie Thompkins as a confidante to Batman. Twice he got rid of Robin. He developed villains like Ra’s al Ghul and his daughter, Talia (who became the mother of Batman’s son); revived another, Two-Face; and turned the Joker from a clownish prankster into a homicidal psychopath.

“He had been watered down, and it works if he is a maniac and is totally unpredictable,” Mr. O’Neil told 13th Dimension, a comics website, referring to the Joker. “It does not work if he … steals groceries!”

Batman’s resurrection as the Dark Knight influenced the two Batman films directed by Tim Burton (“Batman,” in 1989, and “Batman Returns,” in 1992) and the three by Christopher Nolan (starting with “Batman Begins,” in 2005).

“You can argue that the way Denny defined Batman drove all media incarnations,” Paul Levitz, a former DC president and publisher, said in an interview. “We lured Christopher Nolan onto the project by presenting him with ‘The Man Who Falls’” — a comic book written by Mr. O’Neil — “which had an unrevealed piece of Batman’s origin: Bruce Wayne falling into a cave full of bats as a kid.”

Mr. O’Neil was a mentor to Frank Miller, the writer and illustrator who further propelled the Batman franchise in the 1980s with his “Dark Knight Returns” series. At the time, Mr. O’Neil was a DC editor.

Dennis Joseph O’Neil was born on May 3, 1939, in St. Louis. His father, Joseph, owned a grocery store; his mother, Ruth (Noonan) O’Neil, was a homemaker. As a child Denny was a fan of comic books, movies and radio shows like “The Adventures of Superman” and “Jack Armstrong, the All-American Boy.”

But comic books had faded in his life by the time he graduated, in 1961, from Saint Louis University, where he studied English literature, creative writing and philosophy. He went on to serve in the Navy and was aboard the aircraft carrier Champlain during the United States’ naval blockade of the Soviet Union during the Cuban missile crisis in 1962.

After his discharge, Mr. O’Neil was hired as a reporter for The Southeast Missourian in Cape Girardeau, Mo., where he wrote two articles about comic books for the children’s page. They came to the attention of Roy Thomas, a high school English teacher who edited a comic-book fanzine.

In their first meeting, Mr. Thomas opened the world of comics culture to Mr. O’Neil, who was fascinated. When Mr. Thomas took a job at Marvel soon after they met, he urged Mr. O’Neil to take the company’s writer’s test.

“The test was four copies of The Fantastic Four by Jack Kirby,” Mr. O’Neil told the podcast “Epic Marvel” in 2017, referring to the Marvel artist who collaborated with Stan Lee on the creation of the Fantastic Four, Thor, the Incredible Hulk and other characters. “And my task, should I accept it, was to add words to the pictures. Well, OK, who wouldn’t do that?”

Mr. O’Neil passed the test and joined Marvel in Manhattan at its Madison Avenue offices in September 1965. But he was forced out after six months by Mr. Lee, although he continued to write for the company for a while as a freelancer.

“Stan had been only marginally happy with what Denny had written on a couple of superhero stories,” Mr. Thomas wrote in an email. “And then one day he called me into his office and told me that he just didn’t think Denny had his mind on the job.”

To make ends meet, Mr. O’Neil wrote a book on presidential elections and worked at Charlton Comics before moving to DC in 1968. He remained for 12 years. By then, his reputation secure, he was welcomed back to Marvel by Mr. Lee.

As a writer and editor, Mr. O’Neil oversaw Mr. Miller’s work on Daredevil and took over writing Iron Man. He delved into the repercussions of the alcoholism of Tony Stark, the superhero’s alter ego, and the temporary transfer of the character’s armor from Stark to another character, James Rhodes.

Mr. O’Neil gave Stark’s struggles a personal twist based on his own addiction; he considered himself a recovering alcoholic.

Mr. O’Neil stayed at Marvel until 1986, when he moved back to DC, where he was an editor of the Batman franchise until he retired in 2001. He wrote for DC during that period as well, and he continued to contribute after his retirement; his last Green Lantern-Green Arrow story is being published this month.

He also wrote “The Perils of Captain Mighty and the Redemption of Danny the Kid” (2017), an autobiographical novel set in the comic-book world of the 1960s.

In addition to his son, Mr. O’Neil is survived by his stepdaughters, Beth Reuter and Meg Sczyrba, and his brothers, Thomas, Dave, Joseph and Terrence. His marriage to Anne Heaney ended in divorce. His second wife, Marifran (McFarland) O’Neil, died in 2017.

One of Mr. O’Neil’s first assignments at Marvel in the 1960s was Millie the Model, a far cry from Batman. As he told the Comic Archive in 2010, writing about Millie helped him understand the basics of comic-book storytelling — and advanced his early ambition.

“My job was to imitate Stan Lee,” he said. “It was very clear-cut. And I succeeded in that my work was indistinguishable from his.”

George Gene Gustines contributed reporting.