In 1982, Marshall created the comic strip, Buford, distributed through Syndicated News Services, Inc. From 1994 to 2000, he worked with Mark Cullum on the King Features strip Walnut Cove. Marshall's editorial cartoons regularly appeared in the Press & Sun-Bulletin (Binghamton, New York) from 1989 to 2003. In the summer of 2002, he lettered and inked Hagar the Horrible. From February 2001 to January 2003, his daily panel, The U.S. of Play, appeared on United Features Syndicate's website. "I floundered for a couple of years," said Marshall, but he found a new direction in 2002 when he got a phone call from King Features to work on Blondie.

Marshall began assisting Denis Lebrun on Blondie in December 2002. He became the strip's lead artist in May 2005, although he remained uncredited until Sunday, January 7, 2007. Former Cracked cartoonist Frank Cummings is Marshall's assistant on Blondie. Computer technology makes it possible for Marshall, Young and Cummings to collaborate even though they live in three different states. Cummings lives in Birmingham, Alabama, while Young alternates between Vermont and Florida. Young says that Marshall and Cummings are "doing a most wonderful job".

At his Binghamton basement studio, jazz buff Marshall listens to jazz while he draws Blondie. To capture the finely polished inking details seen in Blondie, Marshall works on a Wacom tablet linked to his Macintosh. First he draws a rough, sent to Young for review, and then it's back to the computer for the finished art, delivered electronically to King Features. "It's very deceiving how hard this is to draw," said Marshall in 2007. "I love to draw. I love to see that finished black on white. It's just really primal and just baslc."

Young has nothing but praise for Marshall's work, commenting, "He's been doing such a great job that the Bumsteads and I couldn't be more pleased." Translated into 35 languages, Blondie is published in 2,300 newspapers in 55 countries.

Marshall views Blondie as "a dream job," even though he often works six days a week, ten hours a day. "I definitely feel I'm drawing an American institution," said Marshall in 2007.

Reprints of Marshall's art includes the book Chicken Soup for the NASCAR Soul (Health Communications, 2002). Many of his editorial cartoons appeared annually in Charles Brooks' Best Editorial Cartoons of the Year from 1994-2002.

In addition to the selections in the Best Editorial Cartoons annuals, an Honorable Mention for Editorial Cartoons was received by Marshall in the New York State Associated Press Association Writing Contest for 1996.

Marshall is a member of the National Cartoonists Society. In addition to experimenting with digital photography, he enjoys following his favorite football team, the New York Giants, and creating sports paintings. John and Cheryl Marshall have a son, John.