1833, from noun phrase (1810), from second (adj.) + class (n.). Phrase second-class citizen is recorded from 1942.
The Negro recognizes that he is a second-class citizen and that status is fraught with violent potentialities, particularly today when he is living up to the full responsibilities of citizenship on the field of battle. [Louis E. Martin, "To Be or Not to Be a Liberal," in The Crisis, September 1942]