Assen Diamandiev graduated from the State Academy of Music in 1940. He studied Piano under Professor Dimitar Nenov, Theory of Music under Professor Nikola Atanassov, Professor Vesselin Stoyanov, Professor Assen Karastoyanov, Choral Conducting under Professor Tzanko Tzankov.

He was teacher in Svilengrad. Later he was among the founding members and conductor assistant of the choir of the Ensemble of the Bulgarian Armed Forces, as well as conductor assistant of the Kaval Choir. He also taught at the National Sport Academy (former Higher School of Physical Culture and Sport), conducted the choir of the Plovdiv Singing Society (since 1946), taught at the newly founded music high school in Plovdiv (where he was elected Headmaster from 1951 to 1967), founded the first Bulgarian professional ensemble for traditional song and dance (1948). He was among the founding members of the State Academy of Music subsidiary in Plovdiv (1964) and its first Dean. He was also first Rector of the subsidiary transformed in 1972 into an independent institution and named Academy of Music and Dance Art in 1995. He was promoted Full Professor of Solfeggio in 1971.

Choral songs for various ensembles predominate in his work. He made over 150 arrangements for traditional music choir and composed lots of choral songs as well as instrumental and vocal-instrumental works. The recording company Balkanton released works of his on LP. In 2000 he was awarded the Golden Lyre Prize by the Union of Bulgarian Music and Dance Professionals.

He also wrote solfeggio textbooks, edited and compiled collections of songs and orchestral works, wrote articles, verses, etc.

Works for symphony orchestra:
Concertino for violin and orchestra (1983); Music for orchestra (1989).

Chamber Music:
Little Suite for flute, oboe, guitar, violoncello and bongos (1988).

For string quartet:
Rhodope Miniatures (1967); The Window (1990).

For Trio:
for flute, clarinet and bassoon (1978); Music for Three for percussion (1983); for piano, clarinet and percussion (1992); Piece for two trumpets and drum (1990).

For two violins:
Duet (1975); Three Pieces (1976); Three Pieces (1980).

For two violoncellos:
Duet for two violoncellos (1981).

Concertino for violin and piano (1983);
Emotion for violoncello and piano (1987);
Suite for flute and guitar (1990).

Triptych for solo flute (1974).

For piano:
Easy Pieces (1965); Five Bagatelles (1972); Sonatina (1976); Scherzo for piano for four hands (1976); Three Impressions (1987); Two Pieces (1987); Music for two pianos (arrangement of his First and Second Impression for piano) (1995).

Vocal music:
Seven Songs for flute, violoncello and voice, lyrics by the author and other poets (1995).

Selected books (published in Bulgarian):
Music Dictations Based on Traditional Intonations. Vol.1-3. (Sofia, 1952).

Selected literature on him (in Bulgarian):
Koyumdjiev, Yulian, Polina Koyum-djieva. Assen Diamandiev. Academy of Music and Dance Art Publishers, Plovdiv, 2000.