After election to the board of Aldermen in 1854, Wallach joined the Republican Party and ran against Anti-Know-Nothing candidate James G. Berret for mayor of Washington.

In 1861, Wallach was serving as the president of the board of Aldermen when Mayor Berret was arrested for refusing to take a loyalty oath to the United States, pursuant to emergency Civil War legislation passed by the U.S. Congress.

On August 26, 1861, the Washington City Councils elected Wallach to serve out the remainder of Berret's two-year term.

Wallach's mayoral administration coincided with the District's most drastic period of growth, from a small rural village to a crowded metropolis (thanks to the influx of soldiers and massive federal bureaucracy necessitated by the U.S. Civil War.

During the 1820s, Wallach was a member of the prestigious society, Columbian Institute for the Promotion of Arts and Sciences, who counted among their members former presidents Andrew Jackson and John Quincy Adams and many prominent men of the day, including well-known representatives of the military, government service, medical and other professions.

[9] John H. Brooks, a black member on the school board, pushed for resolutions of memorial for the late mayor, claiming that "the colored race owed him a debt of gratitude.