In order to give him the possibility to concentrate on composition, the Ministry of Education and Cultural Affairs transferred him to positions in the administrations of the Athens Radio and later of the Greek National Theatre.
While there he composed incidental music for 36 plays by Greek and foreign authors.
The other great passions of Georgios Kasassoglou, besides music and his family, were poetry and Ancient Greece.
The antique Greek theatre fascinated him. In close collaboration with the eminent authorities of his time he wrote incidental music for numerous tragedies and comedies of Ancient Greece.
He composed ballet music for the dance ensembles of Rallou Manou, Manon Renieri, and others. Between 1945 and 1955 he worked in close collaboration with the most significant writers, directors, stage designers and actors of the time.
He wrote essays, studies about music and reviews for newspapers and magazines, as well as critical treatises on current musical events.
Avra Theodoropoulou, a renowned music critic of the time, wrote: „Kasassoglou is not the product of a particular school; one could say that he is a creation of his own. The work of Kasassoglou extends over all musical forms except opera. Art song is the form that inspires him the most.