He executed commissions from Newburyport, Massachusetts, to Mobile, Alabama, and worked in Philadelphia, Baltimore, Columbia, South Carolina, and Washington, D.C.

Perhaps most interesting is the range of buildings and machines that Mills designed - from monuments and local courthouses, to prisons and churches, bridges and canals, to rotary piston engines and fireproof masonry vaults - all during a revolutionary era of building technology in America.".

Letter, 27 Mar. 1838, to Hugh Swinton Legare, re congressional controversy over location, design, and material used in the U.S. Treasury fire-proof building in Washington, D.C.; letter, 9 Aug. 1838, Washington, D.C., to Benjamin Ogle Taylor, re Taylor's aid in securing a bank loan for mantel pieces in the new U.S. Treasury building.