In 1876, he began work as an assistant in the Library of Parliament and discovered an interest in botany and entomology.

Fletcher established a national reporting system to help identify and control the spread of insects and weeds harmful to agriculture.

He helped set up measures to control the spread of plant diseases and harmful insects from both within and outside Canada.

Fletcher also initiated the Canadian National Collection of Insects, Arachnids, and Nematodes (CNC).

[1] A federal historical marker reflecting that status was unveiled on November 9, 2017, at the Experimental Farm.