Date of Seniority Royal Navy 18 November 1812
John Johnston was entered in the Navy List of Medical Officers in 1814. 
He was employed as Surgeon Superintendent on the Earl St. Vincent
The Earl St. Vincent departed England on 9th August 1818 and arrived in Port Jackson on 16th December 1818.
Soon afterwards he submitted a request for payment by government for his lodgings while in the colony. He received the following reply: Having submitted to His Excellency The governor, your letter of the 23 inst. soliciting to be provided with lodging money during your stay here, until the 'Shipley' shall be dispatched by which opportunity a passage home is provided for you. I am instructed to inform you that His Excellency regrets that he cannot comply with your requests in either form, as the constant usage here affords no precedent of such allowance (however reasonable) being made
. Colonial Secretary Correspondence. 26 December 1818.
Return to England
John Johnston returned to England in April 1819 on the Shipley with seven other naval surgeons. - Robert Espie, Andrew Smith, William Hamilton, Thomas C. Roylance, Henry Ryan, Morgan Price and John Whitmarsh .
 Navy List
 New South Wales, Australia, Colonial Secretary's Papers, 1788-1825. Copies of Letters sent within the Colony (Ancestry)