While organist and faculty member of Christ Church, Oxford Armstrong combined academic work with practical musicianship, as player and conductor.

[1] The young Armstrong was a chorister at the Chapel Royal, St James's Palace from 1907 to 1910, during which time he sang at the funeral of King Edward VII in Westminster Abbey.

[2] Armstrong remained in France for some months after the cessation of hostilities, not returning to England until 1919; he rented a room with a piano in Cambrai to get himself back in practice.

[1][3] After leaving Oxford in 1922 Armstrong briefly took up an appointment at Manchester Cathedral (assistant organist), during which time he worked with Hamilton Harty.

[1] Armstrong returned to Oxford in 1933 as organist of Christ Church in succession to William Harris who had been appointed to St George's Chapel, Windsor.

His son has written that Amstrong "found the wider academic community deeply congenial, especially the Senior Common Room at Christ Church … For him, Oxford was, and remained, a spiritual home.

"[3] When Allen died, after a road accident in 1946, Armstrong hoped to be appointed to succeed him as professor of music at the university, but the post was given to Jack Westrup.

Supported by his wife Hester, who provided a refuge of comfort and affection for literally thousands of students, he guided the Academy's often shaky fortunes with wisdom, foresight and humanity for the next 13 years.

[2]As well as his concern for the well-being of his students and staff, Armstrong worked to develop links between the RAM and conservatoires of music in other European countries.

[3] His biographer Richard Stoker writes that Armstrong "belonged to the gentle English tradition of Parry, Vaughan Williams, Howells, and Finzi".