[5][6] Herrmann's first legal job after passing the bar was for Wilmington attorney Steward Lynch, for whom he worked until enlisting in the United States Army in 1942.
In 1946, he was discharged as a major and served as an Assistant United States Attorney for three years until he was appointed to the Delaware Superior Court by Governor Elbert N. Carvel, taking office on January 12, 1951.
Because the compensation was too low to pay for his sons' education, Herrmann resigned from that seat on April 15, 1958, to return to private practice with colleague William Duffy.
In his June State of the Judiciary Address (which he established for chief justices), Herrmann called attention to the expansion of the Delaware Supreme Court.
In October, the court expanded from three justices to five when Governor Pierre S. du Pont IV announced that William T. Quillen and Henry R. Horsey would fill the two additional seats.
The new courses he created aimed to teach law students alternative procedures for resolving disputes and how to be better advocates in appellate court proceedings.