Dr. Thomas C. Brainerd was born in 1837 in Pennsylvania, to Rev. Thomas Brainerd, a Presbyterian clergyman. He studied medicine at Jefferson Medical College in Philadelphia for at least the 1859-1860 academic year. He enlisted as an officer in the Union army during the American Civil War and served as a surgeon.

In 1866, he resigned from the army due to his wife's poor health and abandoned medicine altogether. He then began working in the iron business followed by the manufacture of blasting powder. In 1867, he joined the Laflin Powder Company, where his father-in-law Joseph Milton Boies was president, where he rose from the position of agent to department head and then to secretary around 1870. He was also appointed secretary of the board of the Gunpowder Trade Association of the United States in 1872.

He operated his own gunpowder mill near York, Pennsylvania from 1874 to 1876. He immigrated to Canada in 1876 to run the Canadian branch of the Gunpowder Export Company Limited. He purchased the Ontario-based Hamilton Power Company with his friend Lammot Du Pont at the end of 1878 and acted as its president. His company purchased many competitors in Canada. In 1886, Brainerd and four partners including the future Liberal-Conservative Prime Minister John Joseph Caldwell Abbott and the Conservative senator John Hamilton, founded the Dominion Cartridge Company Limited, which dominated the ammunition market in Canada.

He was listed in the 1901 census as living in Montreal. In 1904, he suffered a stroke that partially paralysed him, and his eldest son, Dwight, took over as president of Hamilton Powder Company. He passed away in 1910. The factory he opened in Belœil, QC remained in operation for 120 years.