In 1918 his personal yacht was taken by the United States Navy, becoming USS Wakiva II (SP-160) and was credited with sinking 3 German U-boats during World War I. Harkness sued the US government in 1918 because he did not believe that he was adequately compensated for the value of his yacht.
He died of influenza in 1919, at his home, 270 Park Avenue, New York - a short four years following the death of his father Lamon V. Harkness. In 1918, Harry commissioned the prestigious Schmieg/Hungate/Kotzian furniture manufacturer to exclusively create the Artcase for his Steinway Model B from Steinway & Sons. The piano was discovered in a mansion in 2008 and has been fully restored.
Following Harkness' unexpected death the facility was sold to real estate developers. Following his death, his first wife, Mrs Marie M Cowan (née Harkness née Marbeck), whom he married in 1906 and divorced in 1916, sued for his entire $20 million estate, claiming he was of unsound mind when leaving the estate to his second wife, Mrs Florence Steuber Harkness née Gaines.
Harry was an avid race car driver and won the first "Race to the Clouds", the Mount Washington Hillclimb Auto Race. Harry Harkness led a group of investors who bought the Sheepshead Bay Race Track, a thoroughbred horse racing facility they converted to use as an auto racing track.
Harkness financed the building of many early airplanes. As a noted racer of the day, he was retroactively awarded the 1902 American Automobile Association National Championship in 1951. 1902 was the first year of AAA sanctioned racing. One of Harry's more famous races was against Henry Ford.