Wein’s first professional comics story was “Eye of the Beholder” in DC’s Teen Titans #18 (Dec. 1968), where with co-writer and fellow future-pro Marv Wolfman he introduced the male character Starfire who was eventually renamed Red Star. Wein later began scripting sporadic issues of such DC superhero titles as Adventure Comics (featuring Supergirl and Zatanna), The Flash, and Superman, while continuing to write anthological mysteries, along with well-received stories for the semi-anthological occult title The Phantom Stranger #14–26 (Aug. 1971 – Sept. 1973).
Wein and artist Bernie Wrightson created the horror character Swamp Thing in The House of Secrets #92 (July 1971). Over the next several decades, Swamp Thing would star in DC series and miniseries — including an initial 1972–76 series begun by Wein and Wrightson, and the mid-1980s Saga of the Swamp Thing, edited by Wein and featuring early work by writer Alan Moore. He wrote a highly well-regarded run of Justice League of America (issues #100–118) with artist Dick Dillin.
In the early 1970s, Len began writing regularly for Marvel Comics. Wein’s first superhero work for Marvel was a one-off story in Daredevil #71 (Dec. 1970) co-written with staff writer/editor Roy Thomas. He succeeded Roy Thomas as editor-in-chief of the color-comics line in 1974, staying a little over a year before handing the reins to Wolfman. Remaining at Marvel as a writer, Wein had lengthy runs on Marvel Team-Up, The Amazing Spider-Man, The Incredible Hulk, The Mighty Thor and Fantastic Four, as well as shorter runs on such titles as The Defenders and Brother Voodoo. In 1975, he and artist Dave Cockrum revived the Stan Lee / Jack Kirby mutant-superhero team the X-Men after a half-decade’s hiatus, reformatting the membership. Among the characters the duo created were Nightcrawler, Storm, Colossus, and Thunderbird; Wein had additionally created Wolverine earlier, with artists John Romita Sr. and Herb Trimpe, in The Incredible Hulk. Wein plotted the next two X-Men stories with artist Cockrum. These issues were then scripted by Chris Claremont, who subsequently developed the series into what became, with Spider-Man, one of Marvel’s leading franchises.
At the end of the 1970s, following a dispute with Marvel management, Wein returned to DC as a writer and then eventually an editor. He scripted a long run of Batman and collaborated on Green Lantern with artists Dave Gibbons and Mark Farmer. He also dialogued the mini-series Legends over the plots of John Ostrander and the artwork of John Byrne and Karl Kesel. As editor, he worked on the first mini-series Camelot 3000, and such successful series as The New Teen Titans, Batman and the Outsiders, Crisis on Infinite Earths, All-Star Squadron, and Alan Moore and artist Dave Gibbons’s acclaimed and highly influential Watchmen miniseries.
Wein later wrote a Blue Beetle revival, scripted a revamped Wonder Woman over penciller George Perez’s plots, and created the superhero Gunfire with artist Steve Erwin.
Following his second stint at DC and a move to the West Coast, Wein served as editor-in-chief of Disney Comics for three years in the early 1990s. After leaving Disney, Wein began writing and story editing for such animated television series as X-Men, Batman, Spider-Man, Street Fighter, ExoSquad, Phantom 2040, Godzilla, Pocket Dragon Adventures, Reboot and War Planets: Shadow Raiders. In 2001, he and Wolfman wrote the screenplay “Gene Pool” for the production company Helkon, and later adapted it for a one-shot comic book for IDW Publishing. In September 2004, Wein completed a script for a Swamp Thing feature for Silver Pictures at Warner Bros.. In 2005 and 2006, Wein appeared frequently as a panelist on the Los Angeles theatre version of the TV game show What’s My Line, and in 2006, collaborated with writer Kurt Busiek and artist Kelley Jones on the four-issue miniseries Conan: The Book of Thoth for Dark Horse Comics. He has also scripted the comics series The Victorian for Penny-Farthing Press and has written comic-book stories for Bongo Comics’ TV-series tie-ins The Simpsons and Futurama.