[25] Harrison became a founding member and first president of both the University Club, a private gentlemen's club in Indianapolis, and the Phi Delta Theta Alumni Club.[26] Harrison and his wife became members and assumed leadership positions at Indianapolis's First Presbyterian Church.[27]
Having grown up in a Whig household, Harrison initially favored that party's politics, but joined the Republican Party shortly after its formation in 1856 and campaigned on behalf of Republican presidential candidate John C. Frémont.[28] In 1857 Harrison was elected Indianapolis city attorney, a position that paid an annual salary of $400 (equivalent to $11,633 in 2021).[29][30]
In 1858, Harrison entered into a law partnership with William Wallace to form the law office of Wallace and Harrison.[31] In 1860, he was elected reporter of the Indiana Supreme Court.[30] Harrison was an active supporter of the Republican Party's platform and served as Republican State Committee's secretary. After Wallace, his law partner, was elected county clerk in 1860, Harrison established a new firm with William Fishback, Fishback and Harrison. The new partners worked together until Harrison entered the Union Army after the start of the American Civil War.