Prince Sergei was born on 4 July 1890 in Peterhof, Russian Empire, to George Maximilianovich, 6th Duke of Leuchtenberg, and Princess Anastasia of Montenegro.

Sergei's father remarried six years later, when he caught the attention of Princess Anastasia of Montenegro at her sister Milica's wedding.

George and Anastasia soon married and produced two children, Prince Sergei Georgievich and Princess Elena Georgievna, later Countess Tyszkiewicz (1892–1971).

[1][2] Sergei introduced Alexander Andreyevich Svechin to his step-father Grand Duke Nicholas Nikolaevich, who was known for his religiosity and interest in mysticism.

[citation needed] He never married and was the last undisputed Duke of Leuchtenberg (now living descendants of the Dukes of Leuchtenberg have descended from a morganatic marriage, which is why they lost their dynastic status of the members of the Russian Imperial House and the right to the Bavarian ducal title and style).