After leaving the military, he joined the White Fathers, making his vows in 1991 and being ordained priest in 1992.

[1] MacWilliam underwent officer training at the Royal Military Academy Sandhurst.

[8] Having left the British Army, MacWilliam joined the Missionaries of Africa, a Roman Catholic society of apostolic life, who are also known as the White Fathers.

[1] In December 1991, MacWilliam made his vows thereby becoming a professed member of the Missionaries of Africa (M.

[1][2] In 1994, four members of the White Father mission in Tizi Ouzou, Algeria, were murdered in their home.

[1][10] On 16 March 2017, it was announced by the Holy See, that MacWilliam would be the next Bishop of Laghouat in succession to Claude Rault.