[4][5] He worked as a chemist for Swift and Company in Chicago and as a teacher at Fargo College in North Dakota before returning to university.

[2][6] McCoy published numerous papers on physical chemistry, radioactivity and rare earths.

He was the first person to demonstrate that the alpha-ray activity of a compound is proportional to its uranium content, quantitatively indicating that radioactivity is an atomic property.

[8]: 784  As well, McCoy and William H. Ross clearly identified what came to be known as isotopes as chemically inseparable substances,[8]: 791  a realization that enabled researchers to simplify models of the periodic table.

Studying what would become known as the thorium group, McCoy and Ross verified Otto Hahn's prediction of "mesothorium", an isotope of radium.

McCoy and William C. Moore attempted to use electrolysis to produce a metallic species from tetramethylammonium salts.

It was believed to be a mercury amalgam with the general formula HgN(CH3)4 until 1986, when Allen J. Bard proposed a more compelling explanation for the results.