His work followed trends in Victorian style, evolving from the Greek Revival to Second Empire to Richardsonian Romanesque.

Although widely dismissed as "an obscure draftsman" from Cincinnati, Mullett used his political skills to gain appointment as Supervising Architect in 1866.

[citation needed] He designed fireproof federal buildings across the nation, particularly custom houses, post offices and courthouses.

Responsible for contracting with local architects and/or construction companies to deal with subcontractors, source materials and other matters, Mullett was known as a micromanaging authoritarian with an explosive temper.

[2] Influenced by the 1864–1868 remodeling of the Louvre's Pavillon de Flore by Hector Lefuel and Richard Morris Hunt, Mullett produced six massive fortress-like Second Empire federal buildings in St. Louis, Boston, Philadelphia, Cincinnati, New York and Washington D.C. What was called the State, War, and Navy Building rose near the White House.

Mullett was dogged by accusations of extravagance and subjected to five separate investigations into his ties to the corrupt "Granite Ring".