In 1923 Bruni moved to Moscow. In 1923–1927 he lectured at the faculty of graphic arts at the Higher State Artistic and Technical Workshops (VKhUTEMAS), and in 1927–1930 — at the Higher Art Technical Institute (VKhUTEIN). In 1924–1925 Bruni was a member and exhibitor of society Makovets, in 1925–1926 — society Four Arts. In 1930s he lectured at different institutes: in 1930–1933 — at Moscow State Textile Institute, in 1935–1938 — at Moscow Polygraphic Institute. In 1935–1938 Bruni headed the workshop of monumental painting at Moscow Architectural Institute.

In 1920s–1930s Bruni spent summers in the Optina Monastery not far from Kaluga. These trips were very fruitful for the painter: a lot of drawings and watercolors of 1920s–1930s were done there.

In 1930s he worked in monumental style, created frescoes and paintings for Moscow Central Pioneer House (1935), pavilions of the All-Union Agricultural Exhibition (1938–1939), pavilion of USSR at Paris World Fair (1937), at Moscow Central Theatre of the Red Army (1939–1940).

At the end of 1910s Bruni was engaged in book illustration. He illustrated a lot of books, including book by N. S. Gumilev Mik and Louis (1915), the poem by V. V. Khlebnikov Death’s Mistake (1917–1920), Stories for children by I. Potapenko (1910s), Tales by R. Kipling (1929), Children about animals by O. Perovskaya (1933), Shah Nameh by Firdousi (1930s) and others.

During the World War II Bruni was evacuated to Kazakhstan. In 1944 he returned to Moscow.

In 2000 retrospective exhibition of the painter’s works was held in the State Tretyakov Gallery.

Bruni went down in history of soviet art as a master of drawing, watercolorist and as a book illustrator. At the beginning of his creative development, Bruni was engaged in painting and sculpture; he created portraits, mythological paintings, and concave reliefs. Since the beginning of 1920s, he was engaged only in graphic art: easel painting and book illustration. Watercolor came in the stead of painting.

Bruni created his own style of drawing, which was formed under the influence of the French drawings of the end of the 19th — beginning of the 20th century (he saw them in Paris in 1912), Japanese and China watercolors, and also avant-garde experiments, in which he participated in 1910s.

Works by Lev Bruni are in many Russian museums and private collections. There is a big collection of graphic works of the artist in the Pushkin State Museum of Fine Arts, the State Tretyakov Gallery, and the State Russian Museum.