After his move to Little Rock, Walsh became active in politics. As a Radical Republican he was opposed to slavery and frequently expressed his strong opinions in print. He was actively involved in the fight to admit Kansas as a free state. In 1856 he helped to establish a local chapter of the Kansas Settlers Society which sought to encourage and support the settlement of Kansas by people who opposed slavery. In 1858 he suspected the Little Rock city council was embezzling city funds and in order to gain access to the financial records, he successfully campaigned for election to the council. Once the corruption was exposed he resigned his position.