[2][3] Pitcairn's start in aviation was as an apprentice at Curtiss Aeroplane and Motor Company, in Hammondsport, during the summer of 1914.

In 1923, while acting as the president of Owosso Sugar Company, Harold purchased a Farman Sport for his personal use.

His fleet of planes included his Farman Sport, four Curtiss Orioles, a Standard Aircraft Corporation trainer, and two Martinsyde biplanes with enclosed cabins.

On 14 February 1929, Pitcairn then bought the U.S. rights to Juan de la Cierva's inventions and patents for $300,000.

The deal included the air-mail route, the fixed base operations at New York, Richmond, Greensboro, Spartanburg, Atlanta, and Miami, while Pitcairn retained his Willow Grove flying field, the Athyn factory, and the Pitcairn-Cierva Autogiro Company.

[3]: 159–160 He was awarded the Collier Trophy in 1930 for the "development and application of the Autogiro and the demonstration of its possibilities with a view to its use for safe aerial transport.

"[7] USA President Hoover awarded the trophy on the lawn of the White House in 1931, where a Pitcairn PCA-2 landed as the first aircraft ever.

According to Frank Kingston Smith Sr., "For the first time, a Pitcairn Autogiro would have no stubby wings, no ailerons, no elevators, just a rudder.

On 26 October 1936, Pitcairn delivered his AC-35 to win the Department of Commerce competition for a rotary aircraft capable of flying 100 miles per hour (160 km/h), take off from a 30 square feet (2.8×10−6 km2) area, then fold its wings in a "roadable" configuration.

[3]: 256–300 During WWII, Pitcairn formed the Pitcairn-Larsen Autogiro Company with Agnew E. Larsen to develop seven PA-39s for the Royal Navy.

[3]: 294–298 On 22 July 1943 offered the Army Air Forces Materiel Command a reduced royalty "on machines and equipment supplied to the United States Government by our licensees, we will reduce our royalty from 5% on the basis of fully-equipped machines to eighty-five one-hundredths of one per cent (.85%) of the [government] contract price."

[3]: 323–335 On April 23, 1960 he died from a gunshot to the head at his home CairnCrest in Bryn Athyn Pennsylvania, shortly after a birthday celebration for his brother, Raymond Pitcairn.

[6][2][11][3]: 332–333 More sympathetic sources and the police report said the death was accidental and was caused by a faulty Savage Model 1907 0.32 automatic pistol.