It was painted by Edmund Lewandowski, a leader in the movement known as Precisionism. He taught at the Layton School of Art in Milwaukee and eventually was head administrator. He also has a mural in the Caledonia post office that I've yet to see. In addition to these fine works in Wisconsin he painted some for Minnesota and Illinois, but I'm not sure if they're still around.
In addition to this mural his other publicly visible works included postage stamps and covers for Fortune magazine.
He also completed a gigantic mosaic mural in Milwaukee. Hmmm...might have to track that one down. If you'd like to see other New Deal WPA art, click on the link at the bottom of this post. Linking up to Mural Monday, check out the murals there too. Wisconsin Post Office Mural #5 next week!