Born in England, he emigrated as a child to Ohio. He apprenticed with Isaiah Rogers in Cincinnati and did well enough to become his partner leaving to set up his own practice in 1860. After the Civil War, he moved to Washington. At the time Rogers was the Supervising Architect for the US Treasury Department which oversaw all Federal buildings. Mullett got a job working for Rogers and seems to have undermined him so that Rogers resigned and he ended up in that position in 1866 which lasted until 1874. He managed to churn out over 40 fireproof courthouses, post offices, customs houses and office buildings including the State War and Navy Building which was the largest office building in the world when it was completed in 1888. By that time Mullett had a private practice in New York and had to sue for his fees which he lost on a technicality. He was actually never paid for it. By all accounts he was extremely difficult to work with. He ended up in financial difficulties and died by suicide.
