He was an assistant master at Edinburgh Academy from 1900 to 1903 and at Holt Grammar School before being posted as a professor of natural science (which included the teaching of chemistry) at Muir Central College, Allahabad, from 1905 to 1908, initially in a temporary position (to replace E.G. Hill who was on furlough) which was then extended.

One of his most important findings was in noting the attraction of tephritid flies to methyl eugenol, a component that he identified from several others present in citronella oil.

[11] Howlett also discovered that he could induce Stomoxys calcitrans to oviposit on cotton impregnated with valerianic acid, which is a component of fermenting vegetable matter.

An obituarist in the Agricultural Journal of India noted that he was:[14][15][16][17]... a man of almost childlike simplicity and originality of outlook, and with many interests.

He was a combative apostle of pure research, and his disappearance from the ranks of scientists of this order will be a serious blow to the cause.Howlett assisted Harold Maxwell-Lefroy in writing and illustrating the book Indian Insect Life.

[18] A species of tick, Haemaphysalis howletti, was described by Warburton in 1913 from a pony in Pakistan and in 1962 was found on rodents and birds in Pune, Maharashtra.