John William Mallet FRS (10 October 1832 – 7 November 1912) was an Irish chemist who lived and worked in the United States.

[1] After returning to Ireland and completing his college courses, Mallet went to the United States for the purpose of acquiring information for his father.

In the autumn of 1861 he enlisted as a private in a troop of Confederate Cavalry, but almost immediately was chosen as aide-de-camp on the staff of General Robert E. Rodes.

The following year found him at the Jefferson Medical College in Philadelphia as professor of chemistry, but the unsatisfactory conditions there caused him to return in 1885 to the University of Virginia, which he never again left.

The scientific work of Mallet covered a wide field including general and applied chemistry and chemical mineralogy.

He devised methods for the determination of organic matter in potable water and characterised meteorites and rare terrestrial minerals; in particular, he studied the occurrence of silver in the ash of South American volcanoes.

Mallet also served as expert witness in many court cases involving poisoning, value of iron ore, pollution of river water and other chemical questions.