Hamilton Perkins Cady was born May 2, 1874 outside Council Grove,Morris County, Kansas. After attending Carleton College, Cady received his A.B. in 1897 and his Ph.D. in 1903, both from the University of Kansas in Lawrence, Kansas. He joined the KU faculty as an Assistant Professor of Chemistry in 1899 and reached full Professor status in 1911. In 1921 he was named chairman of the Department of Chemistry, retiring in 1942 as professor emeritus.

In 1905, after a 1903 discovery of a major natural gas well in Dexter, Kansas, Cady and his colleague David McFarland analyzed samples from the well. Through their analysis, they were able to extract helium from the sample, which hitherto had been thought to exist only on the sun and in rare minerals. Cady and McFarland began analyzing samples from other wells around the region and discovered helium was much more plentiful on Earth than previously known.

H. P. Cady married Stella Cornelia Gallup in 1900, with whom he had three children: Ruth Caroline, George Hamilton, and Helen Frances. Cady died in 1943.