Sonntag spent his youth in Cincinnati, to which his family moved in 1823. His father attempted to dissuade him from his early affinity for art by first apprenticing Sonntag to a carpenter, then sending him on an expedition to the Wisconsin Territory, and finally, by apprenticing him to an architect. All these efforts were unsuccessful, and Sonntag, apparently self-taught, tenaciously pursued his goal of a career in painting. He entered his first exhibition, in Cincinnati in 1841, and quickly developed a reputation in landscape painting, while supporting himself for a time as a clerk. In about 1846, Sonntag was employed to "paint the needed dioramas, to make thunder on the drums, to blow for the organist, light the lamps and to make himself generally useful" (Cosmopolitan Art Journal, December 1858, quoted in Spasskey II, p. 162) for the Western Museum in Cincinnati. Also at this time he opened his first studio. His burgeoning career was furthered by several major commissions including; a cycle of four paintings entitled "The Progress of Civilization" (1847), based on Bryant's poem, "The Ages", and painted for the Cincinnati patron, Rev. Elias Lyman Magoon, and in 1852 Sonntag painted sites along the Baltimore Cumberland route for the Baltimore and Ohio railroads. The artist gained exposure as well as skill in the "Grand Manner" by his work on a panorama, "Paradise Lost and Paradise Regained", which was completed by John C. Wolf and exhibited in New York City in 1851, and by his employment as a set painter for Cincinatti's Wood's Theater.
In 1853 Sonntag made his first trip to Europe in the company of fellow artists, Robert S. Duncanson and John R. Tait, a student of Sonntag's. Sonntag remained in Europe for eight months, spending most of his time in Italy. Upon his return to early 1854, he resided in New York, but returned to Cincinnati by mid-year. The following year, Sonntag travelled again to Italy in the hope of remaining there. However, he returned to the United States the following year, and by 1857 was a permanent resident of New York City. Sonntag continued to travel, making several more trips to Italy and sketching trips in America. In the 1860s and 1870s the artist spent summers in New England.
Sonntag began exhibiting at the NAD in 1855 and continued to exhibit annually throughout his life. He was a member of the Artist's Fund Society and the American Society of Painters in Water Colors. He also exhibited at the Western Art-Union, the American Art-Union, the PAFA, and in many major U.S. cities and Paris.
