It did not take long for John and Francis Grant to build up a strong network of clients and business contacts in Jamaica. John, a lawyer, soon was appearing in the Jamaican courts while also acting as attorney for wealthy absentee owners of sugar plantations. John, who married a widow but had no children, was appointed as a judge and elected as a member of the Assembly before becoming Chief Justice of Jamaica.

John acquired Georgia estate on behalf of a Scottish friend and was jointly involved in the administration of Castle Hill estate. Otherwise, there is no record of John having owned a plantation. He did, however, own domestic slaves and, as an attorney, would indirectly have been responsible for the ownership of slaves on various sugar estates. There is evidence that Francis had direct involvement with slaves, not only as the owner of Blackness estate but also as a trader in slaves as a Guinea Factor. This, along with his involvement as attorney for absentee owners, led to the rapid accumulation of wealth.