After becoming, variously, an assistant editor and an associate editor under Joe Orlando on the DC humor series Plop! and the superhero anthology series Adventure Comics, Fleisher, with Carley's script-breakdown assistance, began writing the feature "The Spectre" in the latter title. Beginning with the 12-page "The Wrath of ... the Spectre" in issue #431 (Feb. 1974), Fleisher and artist Jim Aparo went on to produce 10 stories of the supernatural avenger through issue #440 (July 1975) (without Carley's assistance toward the end) that became controversial for what was considered gruesome, albeit bloodless, violence. As comics historian Les Daniels observed, the character, created during the 1940s Golden Age and briefly revived in the late 1960s,

... got a new lease on life after Orlando was mugged and decided the world needed a really relentless super hero. The character came back with a vengeance ... and quickly became a cause of controversy. Orlando plotted the stories with writer Michael Fleisher, and they emphasized the gruesome fates of criminals who ran afoul of the Spectre. The Comics Code had recently been liberalized, but this series pushed its restrictions to the limit, often by turning evildoers into inanimate objects and then thoroughly demolishing them. Jim Aparo's art showed criminals being transformed into everything from broken glass to melting candles, but Fleisher was quick to point out that many of his most bizarre plot devices were lifted from stories published decades earlier."

Writer Harlan Ellison in a 1979 interview praised Fleisher's comics work, while also describing Fleisher and his work as "crazy", "certifiable", "twisted", "derange-o", "bugfuck", and a "lunatic". He also claimed that a Publishers Weekly review called Fleisher's novel Chasing Hairy "the product of a sick mind", and that Fleisher's Spectre run on Adventure Comics had been discontinued by DC Comics because the company "realized they had turned loose a lunatic on the world." While Ellison stated that some of what he was claiming was said "in some humor", Fleisher, saying his "business reputation has been destroyed" and believing he was falsely portrayed as insane, filed a $2 million libel suit against Ellison, publisher Gary Groth and the magazine in which the interview appeared, The Comics Journal. The case came to court in 1986 and resulted in a verdict for the defendants