Lev Abramovich Kassil was born on June 27, 1905, in Pokrovskaya Sloboda, now the city of Engels in the Saratov region, to doctor Abram Grigoryevich Kassil and Anna Iosifovna Perelman, who was initially a music teacher and later a dentist. He studied at a gymnasium, which became a Unified Labor School after the revolution, and graduated in 1923.

Kassil collaborated with the Pokrovskaya children's library-reading room, where various clubs were organized for children and a handwritten magazine was published, of which he was the editor and illustrator.

Since 1923, Kassil lived in Moscow. In 1927, he completed three years of study at the Faculty of Physics and Mathematics at Moscow State University. By his third year, he became fascinated with writing texts and began sending home letters that sometimes reached 25-30 pages. In them, he described his walks around Moscow, new buildings, football, theaters, exhibitions, and museums. His younger brother Joseph, nicknamed Oskya, typed these letters and published them in a local newspaper, where a series called 'Letters from Moscow' appeared. Oskya received fees and spent them on trips to the cinema and treats for friends. Upon learning of this, Lev decided that he could also earn money from literature. Since 1925, he began to pursue writing. In 1928 and 1937, he worked as a journalist, columnist, and special correspondent for the newspaper 'Izvestia'. In 1937, 1941, and 1942, he was the editor-in-chief of the magazine 'Murzilka'.

During the Great Patriotic War, Kassil spoke on the radio, in schools, military units, and enterprises in Moscow and the Urals. In 1947 and 1948, he headed the children's literature commission of the Union of Soviet Writers, and in 1947 and 1949, he led a seminar on children's literature at the A. M. Gorky Literary Institute.

V. V. Mayakovsky first introduced him as a children's writer in the magazine 'New LEF' in 1927. His most famous work, 'Conduit and Shvambrania', based on autobiographical material, tells about the lives of gymnasium students in pre-revolutionary Russia and in 1917 and 1919. After the arrest of his younger brother Joseph in 1937, the book was not reissued for a long time but was not removed from libraries. 'The Goalkeeper of the Republic' became the first Soviet novel about sports. Kassil also participated in the 'LEF' and 'Ref' groups.

He proposed the idea of holding Book Birthdays, which eventually turned into Children's Book Week.

From 1947 to 1970, Kassil lived in Moscow at Kamergerny Lane, house 5/7, building 1, apartment 23, where a memorial plaque is installed.

Kassil passed away in Moscow on June 21, 1970, from a heart attack that occurred while watching the final match of the World Cup. He is buried at Novodevichy Cemetery.

In Engels, there is a House-Museum of Lev Kassil and a street named after him, and a monument in his honor is installed in the square at Freedom Square.