After having served in the United States Navy during World War I, Jaeckle returned to New York to build his private law practice. In 1920, he became clerk of the County Board of Supervisors. In 1927 he became back tax collector for the county treasurer. During this time he cultivated his inside knowledge of the political game. Jaeckle was elected Erie County (Buffalo) Republican Chairman in 1935 and held the position with an iron fist until he resigned it in 1948, ostensibly to devote his full efforts to Dewey's 1948 presidential campaign. During his tenure, he made the county's political organization one of the most powerful in the nation and dominated local elections.