In 1728 he entered the service of Richard Boyle, 3rd Earl of Burlington, at Londesborough Hall, Yorkshire, where he appears to have remained.
He corresponded with Mark Catesby, Emanuel Mendes da Costa, and other members of the Royal Society, and impressed Sir Hans Sloane.
[2] At Wallingfen in the East Riding of Yorkshire, Knowlton discovered the algal "moor balls", known to Linnæus as Ægagropila.
[2] The genus Knowltonia of the order Ranunculaceae, of a number of species of plants indigenous to the Cape of Good Hope, was named after him.
[2][1] The John Knowlton, gardener to Earl Fitzwilliam, with will proved in February 1782, has tentatively been identified as a brother.