Chapman's earliest known credit is as one of the many Golden Age of comic books professionals who contributed to the epic crossover battle between the Sub-Mariner and the original Human Torch in Marvel Mystery Comics #8-10 (June-Aug. 1940), plus an additional story in The Human Torch #5 (Fall 1941; due to a numbering quirk, there was a previous #5, cover-dated Summer 1941). His precise contributions are as lost to history as those of other writers and artists who legendarily jammed on this hastily created confrontation. As described by historian Jess Nevins, Sub-Mariner creator Bill Everett and Human Torch creator Carl Burgos

By the following decade, Chapman was one of at least five staff writers (officially titled editors) under editor-in-chief Stan Lee at Marvel forerunner Atlas, along with Ernie Hart, Paul S. Newman, Don Rico, Carl Wessler, and, on teen-humor comics, future Mad Magazine cartoonist Al Jaffee. Among the titles for which Chapman wrote, beginning in early 1951, are the horror/fantasy series Adventures into Terror, Adventures into Weird Worlds, Astonishing, Marvel Tales, Mystery Tales, Spellbound, Strange Tales, Suspense, and Uncanny Tales; the war titles Battle, Battle Action, Battlefield, Battlefront, Battle Brady, Combat Casey, Combat, War Action, War Adventures, War Combat and War Comics; the Westerns Red Warrior and The Texas Kid; the adventure-drama series Girl Comics, Man Comics, Men's Adventures, and Young Men; the crime fiction series Crime Exposed and Justice; the romance titles True Secrets Love Romances; and such miscellanea as Sports Action, and Speed Carter, Spaceman.

Chapman's last known Atlas works were in comics cover-dated May 1954. His next known credit is a story in the DC anthology title All-American Men of War #18 (Feb. 1955), followed by four years without recorded credits until his name surfaced in two June 1959 DC titles, G.I. Combat #73 and Our Fighting Forces #46. These would be the first of at least 105 war stories he would write in those comics along with Our Army at War, Sea Devils, Capt. Storm, and Star Spangled War Stories.

His and artist Jack Abel's character Sgt. Mule ... whose name, "Millie", meant she was actually not a mule (male) but a hinny (female) ... appeared with various keepers including Private Mulvaney (Our Army at War #149 & 160, Star Spangled War Stories #136); Private Skinner (G.I. Combat #104); and Private Smith (Our Army At War #117).

Chapman's last recorded credit is the story "Paper Bullets", with artist Abel, in Our Army At War #181 (June 1967)

Chapman, who married Bonnie Hano, a production staffer at Atlas, later left comics to become a magazine writer.

Chronological by date of original publication

• Monsters on the Prowl #21 (Feb. 1973)

:"The Drop of Water" (art: Gene Colan), from Marvel Tales #105 (Feb. 1952)

• Crypt of Shadows #4 (July 1973)

:"Locked up!" (art: Carmine Infantino), from Adventures Into Weird Worlds #9 (Aug. 1952)

• G.I. Combat #148 (July 1971)

:"Blind Bomber" (art: Mort Drucker), from Star Spangled War Stories #84 (Aug. 1959)

• DC Special Series #18 (Fall 1979)

:"Frogman Fury" (art: Ross Andru & Mike Esposito), from Our Army at War #102 (Jan. 1961)

• G.I. Combat #146 (March 1972)

:"The Secret Battle Eye" (art: Joe Kubert), from Our Fighting Forces #66 (Feb. 1962)

• Our Fighting Forces #133 (Oct. 1971)

:"No Place for a PT Boat" (art: Joe Kubert), from Our Fighting Forces #76 (May 1963)

• G.I. Combat #144 (Nov. 1971)

:"Straw Pilot" (art: Joe Kubert), from Our Army at War #147 (Oct. 1964)