Loebs's right arm was amputated in infancy because of a cancerous tumor; he writes and draws with his left hand.

His first comics work was for Power Comics Company and on Noble Comics' Justice Machine with Mike Gustovich. His first ongoing series was The Adventures of Wolverine MacAlistaire, about 19th century Michigan frontier life, which he both wrote and illustrated. It was published from 1983 to 1986 by Aardvark-Vanaheim and Fantagraphics, followed by a limited series Journey: Wardrums. He wrote the 31-issue Jonny Quest series published by Comico from 1986 to 1988.

In 1988, he took over writing the recently relaunched series The Flash with issue #15 and continued through #61 (penciled by Greg LaRocque). Meanwhile he wrote Dr. Fate #25-41, and the Jaguar series for DC's Impact Comics imprint. He wrote Epicurus the Sage which was illustrated by Sam Kieth and scripted The Maxx (illustrated and co-written by Kieth).

In 1992 Loebs took over writing the Wonder Woman series begun several years earlier by George Pérez, with pencils by Mike Deodato. During his run from #63 to #100, he created the character Artemis of Bana-Mighdall, for whom he wrote the mini-series Artemis: Requiem. Meanwhile he wrote Hawkman #9-27, penciled primarily by Steve Lieber. In 1996 he had a brief run writing Marvel Comics' Thor. From 1997 to 1999 he wrote Impulse #29-49, penciled by Craig Rousseau. In 1999 he wrote the "V2K" mini-series Brave Old World for Vertigo, penciled by Guy Davis.

His financial situation, following years of limited freelance work and the loss of his and his wife's home in the early 2000s, was publicized in the local newspaper and comics news sites and message boards on the Internet. Author Clifford Meth teamed up with artist Neal Adams to create a benefit auction to help Messner-Loebs. The two also created an art tribute book entitled Heroes & Villains (Two Morrows Press) will all proceeds aiding Messner-Loebs.

His financial situation has improved since then, and he has had a number of works published, including an issue of Green Arrow in 2006, and several pieces in The Three Tenors (Aardwolf Publishing), which he shared credits for along with Clifford Meth and artist Dave Cockrum. A new "Journey" story was included in the one-shot Many Happy Returns (About Comics) in 2008, and IDW Publishing reprinted the original material in paperback. He has done writing for Boom! Studios, including the 4-issue Necronomicon and a story for Zombie Tales. He has done illustration work for the 2007 humor book Chicken Wings for the Beer Drinker's Soul and produces a monthly cartoon for the Livingston [County, MI] Parent Journal. He is in talks with publishers for additional, more substantial new works.

He received an Inkpot Award in 1987.