Born in Dresden, Germany, Otto G. Simonson was educated in public and private schools and at the Polytechnic Institute of Germany.
He emigrated to Hartford, Connecticut, at age 21. He entered the office of supervising architect of U.S. Treasury Department in the early 1880s. He became senior draftsman in that office, but he resigned at the outbreak of the Spanish-American War so that he could serve in the Army as senior captain of Company F, District of Columbia Volunteers. After the war, he returned to the Treasury Department and was appointed superintendent of construction of public buildings. In 1904, Simonson was assigned the task of superintending the work on the new U.S. Custom House in Baltimore (Hornblower & Marshall, Architects). The following year, Simonson entered partnership with Theodore Wells Pietsch. The firm of Simonson & Pietsch lasted until about 1908.
A late project credited to Simonson is the Maryland Casualty Company building in Hampden, constructed in 1921. It is an early example of a suburban corporate campus, the company having moved from downtown. The H-shaped building, now known as the Rotunda, features a distinct bell tower and clock that exists today as a landmark of the Hampden community.