After college, Armstrong worked as a freelancing illustrator. He worked at several advertising agencies, developed animated cartoons, multimedia slide presentations, television spots and print advertising for such companies as RCA and Sears. Armstrong is a three time recipient of the Golden Circle Award for achieving "the highest standards of advertising and selling excellence in worldwide competition."
In 1979, Armstrong teamed up with Funky Winkerbean creator Tom Batiuk to create John Darling. The strip, which starred an obnoxious (fictional) TV talk-show host, ran until 1990. The strip's excellence was recognized by the Northern California Cartoon and Humor Association, which awarded Marvin the Best Comic Strip Award in 1982. In 1996 the National Cartoonists Society presented Armstrong with its Segar Award.