[1][3] He was appointed the Vicar Apostolic of the Highland District and Titular Bishop of Arindela by the Holy See on 27 August 1819, with his residence at Lismore.

As a result of those changes, Bishop MacDonald became the Vicar Apostolic of the newly formed Western District on 13 February 1827.

He did much by his work and conversation to soften down prejudices, and was ever ready to lend his aid in forwarding any scheme which had for its object the advancement of his fellow Highlanders.

[1][2][3] During his time as bishop there came into his possession one of the Scotland's greatest ecclesiastical relics, the Last Chalice of Iona.

It had passed from the possession of Sir Charles Lachlan Maclean, to that of Aeneas created by Charles II., Lord Macdonnell and Aros, and was gifted by Colonel Ranaldson Macdonell of Glengarry, to Bishop Ranald Macdonald, on whose death it came into the custody of his successor, Bishop Scott.