In 1897 Henry Ziegenhein became the thirty-third Mayor of St. Louis.
As Mayor he took an active part in bringing to partial completion the present City Hall. Some offices of the new building were occupied in 1898, though the building was not finally completed until 1904. An Ordinance of 1900 authorized the Mayor to appoint a Factory Inspector to look for violations of State Laws regulating hours of work, sanitary conditions and restrictions on the special election on July 12, 1898. In 1900 the Mayor approved an Ordinance authorizing the start of construction on the City Hospital, which had been damaged extensively in the Tornado of 1896.
Laws passed by the Missouri Legislature have always affected the Mayor's administration of the City's government. A Missouri law of 1899 required the City to appropriate the sum certified by the Board of Police Commissioners as necessary to run the Police Department each year. At the election of 1900 the people of Missouri approved a $5,000,000 bond issue to finance the St. Louis World's Fair of 1904. The population of St. Louis had increased from 451,770 in 1890 to 575,238 in 1900 and it remained the fourth city in size in the United States.