The daughter of a taxi-driver father and a public-stenographer mother,[5] Steinberg was born in Boston, Massachusetts, United States[6] and raised in that city's Dorchester and Mattapan neighborhoods.

Steinberg majored in History at the University of Massachusetts Amherst, where she was a sister of Sigma Delta Tau sorority and received her B.A.

[5] Afterward, while working as a service representative for the New England Telephone Company in Boston, she was a volunteer on Ted Kennedy's first U.S. Senatorial campaign.

[5] In March 1963,[6] Steinberg moved to New York City, and in the career-girl fashion of that era spent some months living at a YWCA and job-hunting through employment agencies.

[9] De facto production manager Sol Brodsky "would come in and set up an extra little drawing board where he would do the paste-ups and mechanicals for the ads," Steinberg said.

[10] She recalled that the "first real Bullpen" — the roomful of artists at drawing boards making corrections, preparing art for printing, and, as envisioned later within Marvel's letter pages and "Bullpen Bulletins", a mythologized clubhouse in which the likes of Lee, Jack Kirby, Steve Ditko, Don Heck and others would be found kibitzing — was created when Marvel moved downtown a few buildings from 655 Madison Avenue to 635 Madison Avenue.

[10] She said that at the time, "[Y]ou were lucky to make $60 a week starting ... and Stan offered me $65, which was a big incentive to sign on!".

[11] In addition to serving as Lee's secretary, Steinberg coordinated with and cajoled artists to turn their work in by deadline, responded to fans' letters, including sending paying members the Merry Marvel Marching Society fan-club kit, and sending artwork to the Comics Code Authority to be examined in order to carry the industry's self-censorship Comics Code seal.

Fabulous Flo Steinberg, as she was known to her public, was as much an institution in Marvel's Second Golden Age as Editor Stan (The Man) Lee himself.

[17][18] Recalling the day of Steinberg's going-away party, Marie Severin observed in 2002: I think the stupidest thing Marvel ever did was not give her a raise when she asked for it because she would have been such an asset to have around later because she's so honest and decisive.

[20] Working for the trade group the American Petroleum Institute, she edited pamphlets and technical manuals for 2 1/2 years, leaving when the organization relocated to Washington, D.C.[21] By this time she had become friends with New York City underground comix cartoonists including Trina Robbins, Kim Deitch, Michelle Brand and Roger Brand.

[21][22] She befriended cartoonists including Art Spiegelman and worked for Gary Arlington's San Francisco Comic Book Company before leaving the city after a year.

There, she recalled in 1984, her Marvel-artist friend Herb Trimpe "had a studio in the upper 80s [of Manhattan] that he wasn't using so I stayed there and went job hunting.

"[21] Steinberg found work running Captain Company, the mail-order division of the horror-comics magazine firm Warren Publishing, staying three years.

[26][27] Critic Ken Jones, in a 1986 retrospective review, suggested that Big Apple Comix and Mark Evanier's High Adventure may have been "the first true alternative comics".

[30] A fictionalized Steinberg starred alongside Stan Lee, Jack Kirby, and Sol Brodsky — all transformed into a Marvel Bullpen version of the Fantastic Four — in the alternate-reality comic What If #11 (Oct. 1978).