Leonid Posen was born in the family of a retired guard captain, in the village of Obolon, Khorol district (now Semenovskiy district, Poltava region), in the Pozen’s patrimonial estate. The future sculptor was tutored at home for primary education, later at the age of fifteen, he was enrolled in the fifth class of the Poltava provincial gymnasium.
In 1872, he graduated from the law faculty of St. Petersburg University. In 1876-1891, Posen served as an assistant to the public Prosecutor in the Poltava district court. Beginning from 1891 worked in the public prosecution office of St. Petersburg district court. In 1912, he became a Senator.
Leonid Posen did not get any specialized artistic education. He was inspired from his early childhood by the painted clay toys in the form of figurines crated by Oposhnya potters. He tried to mold the copies of those toys from the rye bread. He was also fond of drawing. Leonid’s parents wanted him to enter the St. Petersburg Art Academy after he graduated from the grammar school. However the young man did not take seriously his artistic inclinations
Studied sculpture individually. L. V. Posen resumed his art classes upon arrival to Poltava in 1876. At first, he copied plaster figurines of different politicians, using as templates for copying the works of contemporary sculptors: N. I. Liberikh, E. A. Lanceray, P.-J. Maine. Then he began to sculpt from life small portraits of friends and relatives, tried to make complex compositions. For example, in 1877, he molded a wax sculpture group "the Horse-cab Driver".
In 1882, following the advice of G. G. Myasoedov, Leonid Posen presented his sculptural group "On Oxen" at the 10th exhibition of the Society of Wandering Exhibitions. The work of a beginning sculptor was very successful. In several days after the closing of the exhibition, the owner of the art-casting factory K. F. Werfel bought the sculptural group "On Oxen" with the aim to cast it in bronze. Afterwards, for a long time he used to buy works of L. V. Posen for casting and arranged exhibitions not only in Russia but abroad as well: in Italy, England, Germany, Holland and America. At the regular the 11th wandering exhibition, Pozen's works, that had been created previously: "Shinkar" («Pot-house Keeper», "Lyre player", "Ox from Cherkassk", "Foal", "Red-footed Falcon" brought great success to the author.
In 1882-1918, he was a regular participant of exhibitions, organized by the Society of Wandering Exhibitions. In 1891, he became a full member of the Society of Wandering Exhibitions. L. V. Posen was a member of the Society of the Itinerants. From 1894, he was a full member of the Imperial Academy of Arts, and from 1900 a member of the Council of the Imperial Academy of Arts. In 1915, he refused the title of academician in the sphere sculpture "for principal reasons".
Bronze sculptures of L. V. Posen were popular among amateurs of bronze sculptures. Connoisseurs of the works of the Itinerants ordered sculptures from the catalog or bought bronze sculptures directly from the author.
In 1894, due to illness of L. Posen left for Rome, where he undertook treatment and simultaneously studied the museums’ collections. He was elected the academician of the Imperial Academy of Arts (IAA) the same year. In 1900 was appointed a member of the Council of the IAA.< Leonid Pozen was known for his small statuary genre compositions narrating about life and history of Ukrainian people: "Kobzar" (1883), "Migrants" (1884), "The Beggar" (1886), "Cossack on Reconnaissance " (1887), «Scythian » (1889), "Plowing in Ukraine" (1897), etc.
He created a number of portrait busts as well: G.G.Myasoedov (1890), I. F. Stravinsky (1897), N. Yaroshenko (1898, 1899), I. P. Kotlyarevsky (for Poltava school named after the writer), etc.
He worked on the monument to Nikolai Gogol in Poltava in 1914-1915 (opened in 1934). Works of Leonid Posen are stored in museum's collections, including The State Tretyakov Gallery, The State Russian Museum, Kiev Museum of Russian Art, Poltava Art Museum and others.
Leonid Posen was in friendly relations with artists Vasiliy Volkov, Ivan Zaitsev, Nikolai Yaroshenko.