Keith Giffen, a celebrated comic book artist and writer who began his career when comics were still on the fringes of popular culture, in the 1970s, but who rode the superhero wave toward the mainstream with DC Comics’ Justice League and Marvel’s Guardians of the Galaxy, popular franchises that became Hollywood films, died on Oct. 9 in Tampa, Fla. He was 70.

His daughter, Melinda Giffen Frater, said his death, in a hospital, was caused by a stroke.

Mr. Giffen brought new energy, imaginative artistic styles and sly wit to Marvel characters like the Silver Surfer, Nova and Thanos, as well as to DC institutions like Aquaman and the Flash.

In the 1980s, he collaborated with the writer J.M. DeMatteis and the artist Kevin Maguire on offering a fresh take on DC’s Justice League, subverting the traditional superhero melodrama with a heavy dose of humor and reimagining the DC superhero Blue Beetle as a Mexican American teenager named Jaime Reyes. (A feature film version, starring Xolo Maridueña, was released this year.)

He also gave the industry a jolt by introducing absurdist characters that oozed his offbeat sense of humor In the 1970s, for Marvel, Mr. Giffen teamed with the comics veteran Bill Mantlo to create Rocket Raccoon, an animal-kingdom version of a masked marauder known for his acumen with weapons.

The character, whose name was a playful nod to the Beatles song “Rocky Raccoon,” was featured in the hit 2014 Marvel film “Guardians of the Galaxy,” starring Chris Pratt, Vin Diesel, Zoe Saldana, Bradley Cooper and others, as well as in its sequels.

Working for DC in the early 1980s, Mr. Giffen created an intentionally silly character, Ambush Bug, as a wisecracking foil to square-jawed crusaders like Superman. Mr. Giffen later described the character as Bugs Bunny with a teleportation device.

Less wacky, but no less memorable, was Lobo, a villainous interstellar bounty hunter he created in the 1980s with Roger Slifer; Lobo evolved into a popular antihero.

“A couple of times in any given generation in a medium like comics, you get someone who has more ideas in a day than most of us have in a lifetime,” Paul Levitz, a friend and collaborator who became president and publisher of DC, said in a phone interview. “Some of the ideas were just crazy. Some were crazy and wonderful, like a gun-toting raccoon or a bounty hunter chasing people through space on a motorcycle — magnificent enough that they will live on long after him.”

This is not to say that Mr. Giffen himself found all his ideas magnificent. In a 2000 interview, he described the original incarnation of Lobo as “one of these vile, completely unlikable buffoons” and spoke of his dismay “when the damn thing took off.”

Ever the ironist, Mr. Giffen eventually took playful revenge on his creation, subjecting him to parodic but commercially successful story lines, like the one in “Paramilitary Christmas Special” (1991) in which the Easter bunny hires Lobo to kill Santa Claus.

“It was so far over the top,” Mr. Giffen said. “I sent it in to DC just to watch their eyeballs spin.”

Keith Ian Giffen was born on Nov. 30, 1952, in Queens, the elder of two children of James and Rosa (Duncan) Giffen, who both served in the military during World War II. His father was later a maintenance worker, his mother a cook.

The family moved to Clifton, N.J., when Keith was a child. His daughter said he had developed a love of comics, doodling incessantly and drawing his own characters, by the time he turned 8 years old. After graduating from Passaic Valley Regional High School in 1970, he vowed to make his passion a career, despite having no clue how to do so.

“I broke into comics by doing everything wrong,” he said in 2000. “I was working as a hazardous material handler, and I took a week off and said, ‘Hey, I think I’ll break into comics.’ So I just drew up a bunch of pictures and slapped them together. I figured, let me call up the companies and find out how you do this.”

His portfolio piqued the interest of Marvel, which hired him in 1976 to work on a black-and-white story called “The Sword and the Star” with Mr. Mantlo, who would become a frequent collaborator.

By the early 1980s Mr. Giffen was working mostly for DC, where he teamed with Mr. Levitz in 1982 to bring new energy to the decades-old Legion of Super-Heroes series, about teenage superheroes in the 30th century. Their version became DC’s second most profitable franchise in those years, behind Teen Titans, Mr. Levitz said.

“Keith was extraordinarily good at visually embodying what the future might look like on Earth,” Mr. Levitz said of that series. “Of course, an awful lot of that future that was supposed to be a thousand years later is already here.”

Mr. Giffen’s pace scarcely slowed. He continued his prodigious output into his late 60s, working on a wide variety of titles for DC, Marvel and other comics publishers, including Valiant and Image.

In addition to his daughter, he is survived by his sister, Dawn Fabbricatore; his son, Kyle; two grandchildren; and two great-grandchildren. His wife, Anna (Jonasik) Giffen, died in 2015.

Following his death, social media platforms were filled with loving tributes referring to Mr. Giffen as an “icon” and a “genius.”

Mr. Giffen said he never sought such approbation. “I’ve no need to see my name splashed across the covers,” he said in a 1989 interview. “It’s very rare you’ll see me at a convention. I cringe when I hear anybody referred to as a superstar in comics. I think ‘comic book superstar’ is kind of an oxymoron.”