After the Asia Minor Holocaust in 1922, he came to Athens with his family. His interest in art being evident from an early age, in 1929, before even beginning his studies, he published a series of drawings in G. Xenopoulos’ magazine “Η Διάπλασις των Παίδων,” under the pen name “Ακάμας.” From 1931 to 1936, he studied at the Athens School of Fine Arts, under Dimitrios Biskinis and Konstantinos Parthenis, while in 1934, still a student, he designed the sets and costumes for Euripides’ Alcestis, produced by Karolos Koun at the Laiki Skini. His first exhibition was organised while he was still as student, consisting of his early Cubist work at the Atelier in 1930, with the encouragement of Fotos Giofyllis; the following year, he exhibited at the Art Asylum. In 1949, he organised a major solo exhibition at Romvos Gallery, followed by a long period of absence and isolation, interrupted in 1975 by a solo exhibition at Ora Gallery in Athens. He also participated in group shows in Greece and other countries, including the Belgrade International Exhibition of Plastic Arts in 1977, the 1982 Europalia, and the Venice Biennale of the same year, and in 1978, a retrospective exhibition of his work was organised at the National Gallery.
A representative artist of the Generation of the 1930s, Diamantopoulos was inspired by both traditional and contemporary trends, especially Cubism. Focusing on the human figure, his works combine a monumental scale with Cubist models, in which prevail large aplats and a realistic style.