He may have been the son of Alexander Johnston of Long May, “North Britain” who applied for freedom by redemption in the Goldsmiths Company in 1766. The document is noted “jeweller”.
He was assessed to Land Tax from 1767-76 on property in St John Zachary precinct, identified as Carey Lane by other registered silversmiths assessed on the same page. His near neighbours were Jabez Daniell (Grimwade p.483-4) and David Hennell I (Grimwade p.543, 753). The Assay Office was conveniently close by.
There is no name listed against the assessment on the property for 1777 and a new tenant was in place in 1778.
No later information has been found for him.