He was ordained deacon by Bishop James Moorhouse in December 1880 and priest in 1881, He was curate of Christ Church, St Kilda (1881–83) and Christ Church, Hawthorn (1883),[3] first vicar of St Columb's, Hawthorn (1883–94), and Archdeacon of Gippsland (1894–1902).
[5] During his tenure the bishop's lodge was built, St Columb's Hall for the education of clergy established and a cathedral partially completed.
[6] In 1903 he was awarded an honorary DD by the University of Trinity College, Toronto, Ontario, Canada.
On Friday, 11 March 1910, Bishop Armstrong dedicated St Clement’s Church of England Eurobin in the presence of a very large congregation.
The Bishop selected Genesis 28:17 for his sermon: “This is none other than the house of God, and this is the gate of heaven.” He retired[8] on 31 March 1927 and became acting incumbent of St John's Toorak and was president of the Melbourne College of Divinity in 1930.