Marsh didn't serve in the Great War, unlike many other of his classmates, and graduated "on time" in 1920. Even though both his parents were artists, it was only after leaving college that he began to study and practice art seriously.
Somewhere along the line, he focused on what have been called "working class" or "blue collar" subjects, something that became fashionable in intellectual and artistic circles after the Great Depression of the 1930s hit. Rather than featuring a single person as a subject (though he did this to some extent), Marsh tended to feature large groups of people in his more ambitious paintings, placing them in settings befitting their tastes: the beach, Coney Island, burlesque theaters and such. Although it would have been tempting to do so, he avoided strong political statements in most of his Depression-era works (though early in his career he provided illustrations to the leftist New Masses publication). Visual commentary was present in many cases, but Marsh usually downplayed it by casting part of the scene as happy or energetic.
Although he didn't care for modernist art, Marsh incorporated many features of modernism (see my book for details) in his etchings, watercolors and tempera paintings. For example, he distorted the proportions of his subjects somewhat, so they didn't seem quite real. And for some reason, he often liked to depict women as having heavier than average lower legs.
Many of these points and much more can be found in Marsh's Wikipedia entry. What is missing is a discussion of his personal life, though one sentence mentions in passing that he had a wife.
Here are some examples of his work.