Hargrave was born 29 January 1850 in Greenwich, England, the second son of John Fletcher Hargrave (later attorney-general of NSW).
He was educated at Queen Elizabeth's Grammar School, Kirkby Lonsdale, Westmorland and although he had shown ability in mathematics at his English school he failed the matriculation examination in Australia.
He emigrated to Australia with his family, arriving in Sydney on 5 November 1865 on the La Hogue.
He circumnavigated Australia on the Ellesmere after being offered a place. and in 1867 took on an engineering apprenticeship with the Australasian Steam Navigation Company in Sydney.
He later on found the experience of great use in constructing his models.
In 1872, as an engineer, he went on a voyage to New Guinea but the Maria was wrecked, and in 1875 he again sailed as an engineer on William John Macleay's expedition to the Gulf of Papua.
From October 1875 to January 1876 he was exploring the hinterland of Port Moresby under Octavius Stone, and in April 1876 went on another expedition under Luigi D'Albertis for over 400 miles up the Fly River on the SS Ellengowan.
He returned to Sydney, joined the Royal Society of New South Wales in 1877, and in 1878 became an assistant astronomical observer at Sydney Observatory.
He held this position for about five years, retired in 1883 with a moderate competency, and gave the rest of his life to research work.
Among many, three of Hargrave's inventions were particularly significant:
His study of curved aerofoils, particularly designs with a thicker leading edge, the box kite (1893), which greatly improved the lift to drag ratio of early gliders and his work on the rotary engine, which powered many early aircraft up until about 1920.
In his career, Hargrave invented many devices, but never applied for a patent on any of them.
He needed the money but he was a passionate believer in scientific communication as a key to furthering progress.
"Workers must root out the idea that by keeping the results of their labors to themselves a fortune will be assured to them.
Patent fees are much wasted money.
The flying machine of the future will not be born fully fledged and capable of a flight for 1000 miles or so.
Like everything else it must be evolved gradually.
The first difficulty is to get a thing that will fly at all.
When this is made, a full description should be published as an aid to others.
Excellence of design and workmanship will always defy competition".
Of great significance to those pioneers working toward powered flight, Hargrave successfully lifted himself off the ground under a train of four of his box kites at Stanwell Park Beach on 12 November 1894.
Aided by James Swain, the caretaker at his property, the kite line was moored via a spring balance to two sandbags.
He rose 16 feet in a wind speed of 21 mph.
This experiment was widely reported and established the box kite as a stable aerial platform.
Hargrave claimed that "The particular steps gained are the demonstration that an extremely simple apparatus can be made, carried about, and flown by one man; and that a safe means of making an ascent with a flying machine, of trying the same without any risk of accident, and descending, is now at the service of any experimenter who wishes to use it."
This was seen by Abbott Lawrence Rotch of the meteorological observatory at Harvard University who constructed a kite from the particulars in Engineering.
A modification was adopted by the weather bureau of the United States and the use of box-kites for meteorological observations became widespread.
The principle was applied to gliders, and in October 1906 Santos Dumont in a box-kite aeroplane made the first officially recorded flight.
As late as 1909 the box-kite aeroplane was the usual type in Europe.
Hargrave was operated on for appendicitis but suffered peritonitis afterwards and died in July 1915.
He was interred in Waverley Cemetery on the cliffs overlooking the open ocean.