Born in Philadelphia, Pennell graduated from Germantown Friends. He studied art first at the School of Industrial Art (now the Philadelphia College of Art), and later at the Pennsylvania Academy of Fine Arts. Both as a friend and biographer of James Abbott McNeill Whistler, Pennell worked as either a writer or illustrator on more than one hundred books. Pennell frequently collaborated on art and travel books with his wife, Elizabeth Robins Pennell.
For publications such as Century, McClure's, and Harper's, Pennell traveled the world, producing etchings, pen-and-ink drawings, and lithographs of cathedrals, plazas, street scenes, and palaces. He also made panoramic views of major construction and engineering projects, such as the Panama Canal and the locks at Niagara Falls. During World War I, he created a number of important poster designs as a part of Charles Dana Gibson's Division of Pictorial Publicity of the Committee on Public Information, which was organized when the United States entered the war in 1917. Pennell characterized the relationship of government to the arts at the time: "When the United States wished to make public its wants, whether of men or money, it found that art–as the European countries had found–was the best medium."
Therese Thau Heyman Posters American Style (New York and Washington, D.C.: Harry N. Abrams, Inc., in association with the National Museum of American Art, 1998)