He joined the British Indian government to study pearl oyster fisheries in southern Burma in 1906 and then worked in East Africa with the Nyassa Company.
In 1909 an Entomological Research Committee was formed to study ticks of economic importance in Africa.
Simpson and Sheffield Airey Neave were appointed travelling entomologists and he collected mosquitoes, Tabanids, bed-bugs, fleas, lice and ticks in Nigeria in 1910 followed by visits to the Gold Coast and Sierra Leone.
He then took up a position in Turkey to head the Department of Oceanography and Marine Biological Research.
Travelling from Greece on the ship Kyrenia, he was found missing from his cabin, last seen on the morning of 10 November 1936 and presumed drowned.