Son of the wood carver Petros Kossos, he had already become an accomplished sculptor by working with his father before beginning his studies at the Athens School of Arts in 1847. In 1849, on scholarship from the Naval Ministry he went to Rome, where he studied for four years at the St Luca Academy. Finishing his studies, and with a two-year interval in Paris and London, he returned to Athens in 1855 and established his own studio, which became a true school for several sculptors of the following generation.
While still a student, his work was presented in exhibitions at the National Techical University, Athens (1847, 1848). He also participated in the Olympia exhibitions (1859, 1870, 1875), as well as the International Exhibitions in Paris (1855), London (1862) and Vienna (1873).
He pursued allegorical and mythological subjects and produced funerary monuments and busts, particularly of Greek revolutionary heroes, coming to be considered as a national sculptor. He drew his models from classical Greek sculpture and contemporary neoclassicist works, as expected from his studies. Neolassicist principles prevail in his oeuvre: a generalising approach, idealisation of figures; white, smooth surfaces, pupilless eyes, harmonious proportions of individual parts and clear outlines.