William Lamon Harkness (August 8, 1858 – May 10, 1919) was an American businessman and inheritor of a large share of Standard Oil.

He is also a cousin of noted philanthropist Edward Harkness who also benefitted from his father's involvement with Standard Oil.

In 1896, he moved from Cleveland, Ohio to a home at 12 East 53rd Street in Manhattan, New York City.

[1] In August 1911, while in command of his steam yacht Gunilda on Lake Superior, he ran her aground, due to his failure in recognising a requirement for a pilot and then compounding the matter by not hiring tugs at the appropriate time.

[10] He left an estate of $53,439,437,[11] which would be approximately $939 million in 2023 dollars, of which $37,272,254 was stock in Standard Oil.