At the age of fif­teen le Brun en­t­e­red the of­fice of Tho­mas U. Wal­ter, ar­chi­tect of the United Sta­tes Ca­pi­tol. He prac­ticed his pro­fes­sion in Phil­adel­phia from 1842 until 1865, when he re­mo­ved to New York. Among the pro­mi­nent buil­dings de­si­gned by him in Phil­adel­phia are the Ca­the­dral, the Aca­demy of Music, and the Gi­rard Es­tate Buil­ding. The buil­dings de­si­gned by him in New York were the Ma­so­nic Temple, the New York Found­ling Asylum, the Me­tro­po­li­tan In­surance Buil­ding in Ma­di­son Square, and the Board of Edu­ca­tion Buil­ding. He joi­ned the Ame­ri­can In­sti­tute of Ar­chi­tects in 1868 and was twice elec­ted pre­si­dent of the New York Chap­ter. He also ser­ved as pre­si­dent of the Wil­lard Ar­chi­tec­tu­ral Com­mis­sion, or­ga­ni­zed to form a collec­tion of ar­chi­tec­tu­ral mo­dels and casts for the Me­tro­po­li­tan Mu­seum of Art.