Diomedes was born in Athens, Greece to an Arvanite family from Spetses on 3 January 1875.

He studied law and economics in Weimar and Paris and earned a doctorate from the University of Berlin.

It was during his brief term in office (28 June 1949 – 6 January 1950) that the Greek Civil War was concluded.

He was forced to resign amid a scandal involving his Minister for Transport, Hatzipanos.

Along with his wife Julia, Diomidis left part of his fortune to the Greek state for the purposes of establishing a botanical garden in Athens, opened in 1952 as the "Julia and Alexander N. Diomides Botanic Garden".