Young Dennison then immediately returned to Cincinnati, and there commenced the study of law in the office of Hon. Nathaniel G. Pendleton (father of the Democratic candidate for the Vice-Presidency) and Stephen Fales, one of the most eminent lawyers of the West, in his youth a classmate of Daniel Webster at Dartmouth College and always his intimate personal friend. Completing his legal studies and admitted to the bar, he began the practice of his profession in his native city. Soon afterwards he married the beautiful and highly-educated daughter of William Neil, of Columbus (the State capital), a famous and extensive mail-contractor throughout the Northwest, whose name was very familiar to travellers and newspaper-readers twenty or thirty years ago, in the days of stage-coaches, when railroad enterprise was in its infancy at the West.
In 1840 he formed a law-partnership with the once famous, but now infamous, Albert Pike, poet, jurist, and rebel general, Indian savage by adoption and taste, leader of scalping-parties, &c. In the execution of that arrangement he removed to Little Rock, the capital of Arkansas. But the conditions—moral, intellectual, social, and political—by which he found himself there surrounded induced him, after a brief residence and experience, to terminate the connection and return to Cincinnati, where he resumed his professional business. In 1842, at the earnest solicitation of his father-in-law, he removed to Columbus, which became thenceforth his home. He was made solicitor of the Clinton Bank, of that city, then president of the Bank of Columbus; and he finally accepted the entire management and control of all the vast mail-contract and post-road business of Mr. Neil throughout the region between the Ohio and the great lakes.
In politics Mr. Dennison was an original Whig. Throughout the existence of that party organization he was a firm, consistent, and zealously-active member of it. In 1847 he was elected to a two-years term in the Ohio Senate. He next served as president of the Columbus & Xenia Railroad until 1859, when, having been chosen by the Republican party Governor of the State, he resigned his position in connection with corporations. The great rebellion found him commander-in-chief of Ohio. He immediately organized and placed at the disposal of the Federal Government seventy thousand troops, and in offering them gave to George B. McClellan and William S. Rosecrans their first commissions as general officers.