After the Battle of Culloden, Lord Loudoun arranged for him to be appointed in 1746 as an excise officer (Scots: gaudger) in Newmilns, then Stewarton, afterwards Irvine and finally Saltcoats.

Mungo, in the course of his duties, had come across Alexander Bartleymore on the seashore with a cart containing eighty gallons of rum, which he duly seized as contraband.

[6] Another crucial element in the story is that Mungo, in the course of duties, was crossing part of Lord Eglinton's estate on a road when a hare started up and ran through the dyke.

John Brown, tide-officer or tide-waiter (a customs officer who boarded and inspected incoming ships) at Saltcoats, gave evidence that on the day of the earl's death, Tuesday 24 October 1769, he was on duty and walking with Mungo Campbell "They passed through the grounds of Montfodd, and thereafter crossed a burn, which is the march between Montfodd and the earl of Eglintoun's property, and went through lord Eglintoun's ground towards the sea.

Bartleymore stated that Mungo Campbell was one of the two suspected poachers and the earl decided to investigate, leaving his carriage and proceeding down the beach on horseback.

However, he failed and was attacked by the earl's servants who, upon being asked to desist by Lord Eglinton, was tied up and taken to Irvine by cart, then to Ayr, later to Glasgow, and finally to Edinburgh.

[15] After due legal process, Mungo was sentenced to be taken to the tolbooth in Edinburgh, fed on bread and water only, and on 11 April 1770 taken to the Grassmarket where he was to be hanged between 2 and 4pm, after which his body was to be taken for dissection by Dr Alexander Munro.

[20] Lord Eglinton was, it is said, engaged to Jean or Jane, a daughter of the Maxwell family of Pollok House in Eastwood parish near Glasgow, and had been a regular visitor in the months before his intended wedding.

At the hour of his mortal wounding at Ardrossan, a servant at Pollock House was surprised to see Lord Eglinton walking up the stairs to the room reserved for his visits.

[21] Wilson records that "This sad affair, which took place on the grounds between Saltcoats and Ardrossan, was long the topic of discourse in town and country, .."[22] Susanna, Dowager Lady Eglinton, was further devastated by the attitude of many of the Montgomerie estate's tenants who had more sympathy for Mungo Campbell than for the earl, and saw his death as a punishment imposed by heaven, due to the misimprovements of his life and the still more irritating improvement of his estates, his changes of old customs, his interference with old tenants.

[27] Mr Reid of Bonshaw's collection of historical artifacts is said to have included the stirrups from the horse that The 10th Earl of Eglinton was riding when he was shot and killed by Mungo Campbell.

[28] North Ayrshire Council commemorated the incident in 2014 with a plaque on the Montfode Burn bridge and a 'QR' linking to web-based information about Mungo Campbell and Lord Eglinton.