William was a younger son of Samuel Backhouse Esq. of Swallowfield Park in Berkshire. He entered Christ Church College, Oxford, as a commoner, in 1610, at the age of seventeen, but left the university without taking a degree. At length, settling on his patrimony, he devoted his time to the study of the occult sciences, became a renowned alchemist, Rosicrucian and astrologer. He gave great encouragement to those who were addicted to similar pursuits, especially Elias Ashmole, whom he adopted as his son, and to whom he freely imparted the arcana of his mysterious lore. The subjoined laconic entries in Ashmole's diary show the intimacy of the friendship subsisting between them; 26th April 1651: "Mr. William Backhouse of Swallowfield, in the the county of Berks, caused me to call him father thenceforward." 10th June 1651: "Mr. Backhouse told me I must now needs be his son, because he had communicated so many secrets to me." 10th March 1652: "This morning my father Backhouse opened himself very freely, touching the great secret." And finally, under date 13th May 1653, Ashmole writes: "My father Backhouse lying sick in Fleet Street, over against St. Dunstan's church, and not knowing whether he should live or die, about eleven of the clock told me, in syllables, the true matter of the Philosopher's Stone, which he bequeathed to me as a legacy." It is almost superfluous to add that no hint is given as to the nature of this wonderful secret. Backhouse died at Swallowfield on 30th May 1662. He married Anne the daughter of Brian Richards of Hartley Westpall in Hampshire, by whom he had two sons (who predeceased him) and a daughter, Flower, who married, firstly, William Bishop (d. 1661) of South Warnborough (Hampshire); secondly, her father's kinsman, Sir William Backhouse, Bart. (1641-1669); and, thirdly, Henry Hyde (1638-1709), the 2nd Earl of Clarendon.

Backhouse left a number of interesting manuscripts: 'The pleasant Founteine of Knowledge: first written in French 1413, by John de la Founteine of Valencia in Henanlt;' translated into English verse in 1644; A translation of 'Planctus Nature: The Complaint of Nature against the Erroneous Alchymist, by John de Mehung'; 'The Golden Fleece, or the Flower of Treasures, in which is succinctly and methodically handled the stone of the philosophers, his excellent effectes and admirable vertues; and, the better to attaine to the originall and true meanes of perfection, inriched with figures representing the proper colours to lyfe as they successively appere in the pratise of this blessed worke. By that great philosopher, Solomon Trismosin, Master to Paracelsus' a translation from the French. Wood adds that "he was also the inventor of the "Way wiser" in the time of George Villiers, the first Duke of Buckingham." This was a kind of pedometer for attachment to a coach by means of which the distance travelled could be determined.