He painted and studied episodically with Tadas Daugirdas (1908–1910), local painters while serving in the army in Kiev (1911–1917), and Fedosejevo during evening drawing courses (1917–1918) in Saint Petersburg.
Šimonis created about 2,000 works of art – landscapes, portraits, fantasy paintings, as well as ex-libris, posters, proposals for postage stamps, and decorative panels in Birštonas (1938).
His individual exhibitions were held in 1925, 1926, 1927, 1929, 1949, 1958, 1967, 1972, 1977 in Kaunas, 1925 in Washington, D.C., Boston, Chicago, 1926 in Riga, 1927 in Klaipėda and Paris, 1968 and 1975 in Vilnius.
During this period, his works exhibit the growing tendency to divide things and space into geometric segments.
Notable works: "Foliage" (1923), "Towers" (1928), "Fantasy", "Thirst" (1926), "Forest Fire", "Princess", "Composition", "Lights" (1926), "In Fog" (1927), "Fantasy Landscape" (1930), "Candles" (1931), "Living Stones" (1935), "Landscape with Wayside Shrine", "Girl with Flowers" (1936), "Into The Space" (1958), "Breaking Morning" (1968).
[2] He also painted portraits of actors Ona Rymaitė, Unė Babickaitė, Teofilija Vaičiūnienė, writers Vydūnas, Vincas Mykolaitis-Putinas, Vincas Krėvė-Mickevičius, Juozas Tysliava, Liudas Gira, Kazys Binkis, historian Simonas Daukantas, artist M. K. Čiurlionis, and others.