“Fabulous Flo” Steinberg. (courtesy of Marvel)

AT MARVEL COMICS in the early ’60s, as the Fantastic Four and Spider-Man exploded into the pop-culture consciousness, the Manhattan publisher had only two full-time office staffers. One you’ve heard of: editor in chief Stan Lee. But the other one was crucial in her own right: Florence Steinberg, who was an assistant and, as a Marvel presence, so much more.

“Flo Steinberg was my first secretary at Marvel/Timely,” Lee told The Post’s Comic Riffs. “I dubbed her ‘Fabulous Flo’ for good reason. Nobody cared more about her job or the people she worked with than Flo.”

“She spoiled me for future secretaries,” Lee continued about the then-recent University of Massachusetts at Amherst graduate. “To most others, it was just a job. To Flo, it was her life’s work.”

Steinberg, who died Sunday at age 78, according to Marvel, has “always been the heart of Marvel and a legend in her own right,” the publisher said in a statement. Indeed, at Marvel, she was an institution.

She had suffered a brain aneurysm and metastatic lung cancer, according to friend Larry Hama, the veteran comics creator.

“Stan Lee once referred to Flo Steinberg as ‘a little sunbeam,’ and that’s what she was, no matter her age,” Tom Brevoort, the longtime Marvel editor, told The Post. “Bright, fiery, luminescent, lighting up a room — and hot to the touch if you got too close.”

Steinberg became known to readers because she replied to fan mail and published letters pages, but she left Marvel in the late ’60s, reportedly over a pay dispute. She started a small indie press, published her independent comic book, “Big Apple Comix,” in 1975 and became a comics-convention fixture.

“She wasn’t particularly a ‘comic book person,’ ” Brevoort said, “but she absolutely was a ‘comic book people person,’ and so it’s no surprise that she orbited the field in a variety of roles for more than 50 years.”

More than a decade ago, Steinberg returned to Marvel, where, until recently, she was still working as a proofreader, Marvel executive Ryan Penagos wrote on Twitter.

“She was a very special part of Marvel,” Brevoort said, “one that cannot be replaced or replicated, only remembered.”

“The whole comic-book industry knew and loved ‘Fabulous Flo,’ ” Lee says, “and her passing is truly a great loss.”