Garthwaite was born October 3, 1854 in Brooklyn, NY, to Charlotte Hatfield Garthwaite and Harry Garthwaite. The family came to California (via the isthmus of Panama) in 1856.
Garthwaite was a founding member of the Athenian Club. 1
H. P. went to South Africa in 1896, where he worked with Charles Butters. He left South Africa in 1899 at the time of the Jameson Raid (before the Boer War broke out) and went to San Salvador in Central America. He continued to work in mining there, and was a confidential adviser to the president of El Salvador.
Garthwaite died in March, 1911 of 'brain swelling', with his wife Virginia Dupont Garthwaite and son John Gilbert Garthwaite at his side. H. P. left everything to his son, in a trust to be held by W. W. Garthwaite until the boy came of age. 2
See also: Garthwaite family