[4][5][6][c] Theobald lectured at the College at Wye until 1920 when he became advisory entomologist to the Board of Agriculture for the South-Eastern district of England.

With this support, Theobald wrote A Monograph of the Culicidae, or Mosquitoes (in five volumes, 1901–1910) for the Colonial Office and the Royal Society.

[1] The journal Nature notes that he had formed "probably the finest collection in existence of insects of economic importance, showing the various stages and damage done by these pests".

[4][e] He was awarded honorary membership of:[4][13][14]Few people had played such a leading part as Mr. Theobald in the modern development of tropical medicine and sanitation.

Theobald was made an Officer of the Imperial Ottoman Order of Osmanieh by the Egyptian Government in 1907[16] and was awarded the Mary Kingsley Medal by the Liverpool School of Tropical Medicine in 1913.