Anthony M. de Witt, architect, was the son of Dirk de Witt, a leading architect of Dordrecht, the Netherlands, and his wife Geertruida van Velthuizen. He was educated at the high school in Dordrecht, served his indentures with a local architect, and then joined the office of his father. On completion of his theoretical and practical training he was invited in 1876 by President T.F. Burgers of the South African Republic to come to the Transvaal to work on the proposed Lebombo Railway to Delagoa Bay. De Witt waited for some time in Potchefstroom, but the project was postponed indefinitely. Meanwhile he was a regular contributor of articles to the Volksblad and the Cape Times. In 1880 he settled in Cape Town to practice as an architect and during the next three decades designed many buildings, including a large building for Messrs. Lennon Ltd on the corner of Adderley and Strand Street, an enlargement of the Drill Hall, the Commercial Union Buildings, Collonade Buildings, International Hotel, Metropole Hotel, Victoria Cottage Hospital, churches in Wepener and Paarl, and many residences. He introduced the half timber style and the continental renaissance style for residences, and the basement and Mansard roof in public buildings. He also introduced to South Africa the American steel frame construction method for warehouses, the first being an eight-story building for Messrs. J.W. Jagger & Co. in St George Street. This work led to his recognition as a pioneer architect-engineer of the Cape Colony.
De Witt was declared insolvent in 1882 and again in 1911, and was chairman of the Fairfield Brick and Land Company which was declared insolvent in 1900. He enjoyed music and gardening as hobbies and was a member of the South African Association for the Advancement of Science from 1903 for some years. In 1910 his wife, Anna Brandt, returned to Europe for health reasons and died there in 1912. Under the pseudonym Anna de Blanc she wrote My experiences of life in South Africa (Wiesbaden, 1912). De Witt's son Gerard (born 1884) also trained as an architect and joined his father's firm.