Oxford and Royal Academy of Music
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.
He was also conductor of the Oxford Orchestra and Oxford Bach Choir, and president of the Musical Club and Union. In 1939 the governing body of Christ Church elected him a student (the equivalent of a fellowship in other colleges), the first organist to receive that honour. 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." 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.
In 1955 Armstrong was appointed principal of the Royal Academy of Music. The Times later commented on this period:
It was entirely typical of Armstrong that at this stage, aged 57, he should take up playing the double bass, receiving lessons from one of his own staff. 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.
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.
In addition to his RAM duties, Armstrong was prominent in other aspects of British musical life. He was senior adviser to the Delius Trust, chairman of the Royal Philharmonic Society, a member of the Countess of Munster Trust, a member of the board of directors of the Royal Opera House, a Governor of the Old Vic-Sadler's Wells Foundation, chairman of the governors of the Central Tutorial School for Young Musicians (founded in 1962), now renamed the Purcell School, and chairman of the Musicians Benevolent Fund.
Armstrong was knighted in 1958 and retired in 1968, when he was 70. He and his wife moved to the village of Newton Blossomville in Buckinghamshire, close to the Bedfordshire border. There he formed and conducted a village choir, which won several prizes in music festivals. After the death of his wife in 1982, he moved to Olney, a few miles away, sharing a cottage with Rosemary Rapaport, his former colleague from the RAM. Robert Armstrong recalled that in Olney, when already ninety years old, Armstrong "was delighted to be sought out, as 'the old man in East Street who plays the organ', to play for Mass at the local Roman Catholic church for six months."
Armstrong died at his home in Olney at the age of 96.