By 1827 he was serving as a Justice of the Peace and in 1835 Hansard lists Melville as King's Advocate and Registrar of the Vice Admiralty Court in Freetown, Sierra Leone.
Melville's own correspondence with Lord Aberdeen contains a number of detailed and vivid descriptions of the struggle to suppress the transatlantic slave trade and release its victims.
A letter of 27 April 1844 is typical: "The Santa Anna cleared out from Bahia on 24 December last, with slave provisions and equipment, and a small quantity of tobacco and other goods on board, and proceeded direct to Lagos, where she arrived on the 5th of February, when one of her officers, according to the usual custom, seems to have gone on shore to arrange about a cargo of slaves, the vessel putting to sea again in the meanwhile and cruizing [sic] about to avoid risk of capture.
On the 4th of April, having lost in the interim by death several of her human cargo, she was fallen in with, to the northward of Ascension, by Her Majesy's [sic] brig HMS Rapid, and, after a chase, captured; most fortunately for the lives as we well as the liberties of the slaves, the quantity of water found on board being very deficient, and barely lasting until the arrival of the vessel at Sierra Leone... No flag nor any official papers were found in the vessel to indicate her nationality; but that difficulty was sufficiently obviated, and the Brazilian character of the vessel, and the traffic in which she was engaged, clearly established by the admissions of the witnesses, and the evidence afforded by two log-books and a list of the cargo of slaves which had been discovered on board; her condemnation consequently took place as above mentioned, the surviving slaves, 267 in number, (one having died here before adjudication,) being at the same time emancipated."
(M. L. Melville to the Earl of Aberdeen 27 April 1844 in "Correspondence with the British Commissioners at Sierra Leone, Havana, Rio de Janeiro, Surinam, Cape of Good Hope, Jamaica, Loanda, and Boa Vista Relating to The Slave Trade from January 1 to December 31, 1844 inclusive" HMSO[7])In 1840, Melville married Elizabeth Helen Callender, daughter of Randall William McDonnell Callander (died 1858), of Craigforth House Stirlingshire and Ardkinglas Castle, Argyle.