Biographer of Ignatius Sancho, the African writer whose letters were published in England in 1782. Jekyll was the only son of Edward Jekyll, a captain in the Royal Navy. Details concerning his place of birth are uncertain. He studied at Christ Church, Oxford, left for France upon completion of his studies in 1774, and was called to the Bar at Lincoln's Inn in 1788.
Jekyll may have met Sancho during this period, but there is no confirmation of this. In fact, information regarding their relationship is scarce and is left to much speculation. However, one piece of evidence affirms that Jekyll and Sancho did indeed meet and had some form of connection that extended beyond the purely professional. A letter written around 1803 by Sancho's son William to Jekyll, suggests that Jekyll was generous to the Sancho family:
To Joseph Jekyll Esq M P From ...