Eisner studied philosophy, but then became a journalist in Marburg. From 1890 to 1895, he was contributing editor of the "Frankfurter Zeitung", during which time he wrote an article attacking Kaiser Wilhelm II, and for which he spent nine months in prison. Eisner was always an open republican as well as a Social-Democrat, joining the SPD in 1898, although for tactical reasons, German Social-Democracy, particularly in its later stages, rather cold-shouldered anything in the shape of republican propaganda as unnecessary and included in general Social-Democratic aims. Consequently, he fought actively for political democracy as well as Social-Democracy. He became editor of "Vorwärts" after the death of Wilhelm Liebknecht in 1900, but in 1905 was called upon to resign by a majority of the editorial board, which favored more orthodox Marxists. After that, his activities were confined in the main to Bavaria, though he toured other parts of Germany. He was chief editor of the "Fränkische Tagespost" in Nuremberg from 1907 to 1910, and afterward became a freelance journalist in Munich.