Dinsmore worked as a store clerk and later a traveling salesman for a St. Louis firm. Deciding to become a lawyer, he persuaded Samuel N. Elliott of Bentonville to proctor him. In 1872 Governor Elisha Baxter appointed him the eleventh Benton County Circuit Court clerk. He resigned in 1874, when he was admitted to the bar, and the following year, he moved to Fayetteville, where he entered into a law partnership with David Walker. In 1878, he was elected prosecuting attorney for the Fourth Judicial District, and served until 1884.