Wallgren's cartoons appeared in every issue of The Stars and Stripes, poking fun at army life, satirizing the absurdity of army regulations, and highlighting the differences between the army brass and the frontline soldier.

Numerous trips to the war front gave him material for his cartoons and first-hand experience about what soldiers considered humorous.

His popular cartoons were collected in "Wally: His Cartoons of the A.E.F.," published by the newspaper in early 1919, which was being sold for the benefit of the War Orphan Continuation Fund. The un-numbered pages, when folded out, are about 16" X 7".

Following the war, Wallgren worked as a cartoonist for the American Legion Magazine. In 1933, with former editor John Winterich, he published "The A.E.F. in Cartoons."