Nowlan first came to the industry's attention in the early 1980s via illustrations in the fan press, most notably The Comics Journal and Amazing Heroes.
Nowlan's first published work for Marvel Comics was Doctor Strange #57 (Feb. 1983). He has worked for DC Comics and other comics publishers. He contributed to the adult Penthouse Comix. In 1992, he inked the Batman: Sword of Azrael miniseries which introduced the character Azrael. He drew the short story "The Castle" in Vertigo Jam #1 (Aug. 1993) which featured the Sandman and was part of the "The Kindly Ones" story arc. One of Nowlan's prominent contribution to comics is the creation of Jack B. Quick with writer Alan Moore. This character appeared several times in Tomorrow Stories under the America's Best Comics imprint.
Although the majority of his work is as an inker, he has provided both pencils and lettering for various comics. He is a noted cover illustrator. Nowlan contributed character designs to Batman: The Animated Series, most notably The Penguin, The Mad Hatter and the Man-Bat.
Nowlan has described himself as a "finisher" rather than an inker, although only in specific reference to work "where you see too much of me", and has expressed an ambivalence towards this role, saying "it's not the right way to ink someone else's pencils".
His style gives a strong emphasis towards both facial expression and posture, and in neither case is he constrained by the conventions of the comic-book hero, and his protagonists are often depicted with awkward expressions or body postures.
Steve Gerber's posthumous Man-Thing story The Screenplay of the Living Dead Man, with art by Nowlan, originally planned as a 1980s graphic novel before being left uncompleted by the artist, was revived in the 2010s and appeared as a three-issue miniseries cover-titled The Infernal Man-Thing (Early Sept.-Oct. 2012). The story was a sequel to Gerber's “Song-Cry of the Living Dead Man” in Man-Thing #12 (Dec. 1974)
Inkwell Award for Favorite Finisher/Embellisher (2008)
Inkwell Award for The Joe Sinnott Hall of Fame Award (2011)
Batman: Sword of Azrael
Batman: The Animated Series