After the war, Parrott became a lecturer at Birmingham University from 1946 to 1950. After about 1951, Parrott's compositions became deeply influenced by his interest in the Welsh landscape and culture, when he was appointed Gregynog Chair of Music at Aberystwyth until he retired in 1983,
In 1955 Parrott became a founder member of the Guild for the Promotion of Welsh Music. That year he revived the Gregynog Festival, originally held from 1933 until 1938 by the art patrons Gwendoline and Margaret Davies. He ended it in 1961, but the festival, revived again in 1988, has continued.
Amongst Parrott's writings is The Spiritual Pilgrims, a book on the Davies sisters. He also wrote a book on Elgar for the Master Musicians series. A leading authority on Elgar, Parrott was elected a vice-president of the Elgar Society in 1973.[2] He was also a vice-president of the Peter Warlock Society. Parrott wrote a study of the piano music of Cyril Scott. In 1994 he published The Crying Curlew for Peter Warlock's centenary. His autobiography Parrottcisms appeared in 2003. He died in Aberystwyth, and his funeral was held in St Padarn's Church, Llanbadarn Fawr, where in 1985 he donated a stained glass window entitled "Music in Praise of the Lord".
Aphorisms and Arias of Death and Life for soprano, baritone and piano (1996)
Nothing like Grog for voice and piano
Choral
Psalm 91 for bass solo, chorus, semi-chorus and orchestra (1946); Biblical words
Carol plygain (Dawn Carol) for mixed chorus and piano (1958)
Can mewn bar salwn (Song in a Saloon Bar), part-song for male voices (tenor, baritone, 2 basses) and piano (1963); words by A. S. J. Tessimond; Welsh words by T. H. Parry-Williams
The Song of the Stones of Saint David's for chorus and organ (1968)
Offeren yn arddull canu gwerin (Welsh Folk-Song Mass) for unison chorus, organ or piano and percussion (1974)
Surely the Lord Is in This Place, introit for mixed chorus a cappella (1977)