The original Newton County was formed by statute on February 7, 1835, and was a roughly square area some 30 miles on a side, encompassing what is now the northern half of the county, the northern half of Jasper County, and a large section to the north. The northern border was cut back to the Kankakee River on February 1, 1836, with all land north of the Kankakee River going to Lake and Porter counties. The county was abolished and combined with Jasper County in 1839. On December 8, 1859, the county was re-created and the borders were redrawn to essentially their current state.
Newton County is named after Sgt. John Newton, who served under Gen. Francis Marion, the "Swamp Fox", in the American Revolutionary War. It is adjacent to Jasper County, which was named after Sgt. William Jasper, whose story is similar. At least four other states, Georgia, Mississippi, Missouri and Texas, have adjacent Jasper and Newton Counties, as though these two were remembered as a pair.
Newton County was the last county to be organized in Indiana.