During his lifetime, David Gilmour Blythe was regarded in Pittsburgh as a talented eccentric, but he was virtually unknown elsewhere. Today he is recognized as nineteenth-century America's leading satirical genre painter.

Pittsburgh in the 1850s was an affront to Blythe's Scottish Presbyterian morality. While most of his contemporaries painted idealized, happy, healthy children, Blythe squarely confronted and castigated the evils that surrounded him, portraying sickly, ill-fed, ill-tempered, and almost imbecilic young delinquents. He was similarly unflinching in his treatment of Pittsburgh's citizenry in Post Office.

Post Office shows part of the exterior of the old Custom House and Post Office building that once stood at Smithfield Street and Fifth Avenue in downtown Pittsburgh. At the center of the composition, men and women fight for a place at the narrow General Delivery window. The press of the crowd destroys the normally dignified appearance of mid-nineteenth-century costume: a woman's pink crinoline assumes a fantastic balloonlike shape, and the man next to her has his hat crushed and the seat of his trousers ripped in an attempt to reach the window. To the left of the melee, a street urchin deftly picks the pocket of a man absorbed in a letter; at the right, a markedly sleazy character holding a carpetbag surreptitiously examines an unsuspecting man's correspondence.

While Post Office has generally been regarded as a simple, amusing genre piece, free of obscure topical allusions and Blythe's habitual bitterness, the painting makes a serious satirical comment. The Neoclassical bust over the delivery window alludes to the idealism and dignity of the American past, while the indifferent newsboy on the steps symbolizes the squalor of contemporary urban life. In pairing these figures, Blythe contrasts the noble ideals of the nation's founding fathers with the greed, self-interest, and venality he sensed in his own times.