Ellsworth taught school in Howell during the winter and studied law until he was admitted to the bar in 1848. He commenced practice in Howell and, in 1849, was appointed by Michigan Governor John S. Barry as prosecuting attorney of Livingston County. He moved to Montcalm County and settled in Greenville in the spring of 1851 and became the first practicing lawyer in the area. He was elected to the Michigan House of Representatives in 1852 and served a single two-year term. He was twice elected prosecuting attorney of Montcalm County, serving from 1853 to 1857. He had been a Democrat until the Kansas-Nebraska Act in 1854 and the resulting violence sparked the formation of the Republican Party in 1856.