The "Onion House", so named because of its onion shaped dome, was constructed by Thomas Lenehan in 1902. Mr. Lenehan was a Delmont citizen and active carpenter in the community. He built the house to serve as a private residence after completing the Delmont school building. In 1915 Dr. Thomas Arnold established residence in the home.
According to the March 17, 1917 edition of the Delmont Record Dr. Arnold then converted the lower level into a clinic, and the upper level into a hospital. "This institution is prepared to care for eight patients, and giving them all the care they could get in any hospital. The services of the best nurses, skilled medical and surgical attention, and all the comforts of well heated and ventilated rooms in winter and airy cool rooms in summer. The building is electric lighted, is fitted with hot and cold running water, bath, lavatory and toilet and so forth. Each patient is given individual service of meals. Everything is delightful clean and cheery. The building is fitted with a modern operating, where facilities and conveniences count for much."
The article concludes, "The right man is behind it, the man who has thrown his whole heart and ambition into the profession , in the hopes that he may fill his mission in the world towards humanity. It goes without saying that Delmont and the people of the community are proud of Dr. T. Arnold and wish him every success in the Delmont Hospital."
Charles Sroka purchased the home in 1920 and owned it until 1939. Since then the house has been used for several purposes including a beauty shop, duplex, women's clothing shop, and a private residence. As of Aug. 2015 it is part of the museum complex of the Historical Society of Delmont.
Owners since 1939 have been: L. Eirenberg, 1939-1942; Jay and Nellie Hovey, 1942-1946; Jacob and Martha Lindeman 1946-1956; Henery Bitterman, 1956-1964; John and Salma Peters, 1964-1976; Herb and Edna Bierwagen, 1976-1983; Russ and Verla Lehman, 1983-1990; Dick and Earla Strid, 1990-1993; Verla Lehman 1993-2000; Robert and Jan Schuh 2000- 2015; Leo Holzbauer, June 2015 - Aug. 2015; The Historical Society of Delmont, Aug. 2015 - now.
Verla Lehman placed the "Onion House" on the National Register of Historic places in 1987. The South Dakota Historical Society was interested in the house because of its unique characteristics for a small Midwestern town of South Dakota. Those characteristics include an irregular roof line, a wrap-around porch and the distinctive side tower with an onion shaped dome. This dome is believed to be the only residence in the state that contains this architectural structure possibly made of lead or some other pliable metal.