McMillan was a US Senator from Michigan who led the Senate Park Commission in creating a new design plan for Washington's public spaces, including the National Mall. The work and plan of the Senate Park Commission is often referred to by McMillan's name, because he worked very closely with architects and artists appointed to the commission. McMillan died in office in 1902, and would not see his work implemented on the Mall.
James McMillan
Library of Congress Prints and Photographs Division. View original photograph.