Illinois Route 142 is a major arterial road in southeastern Illinois. It runs from near the town center of Equality at Calhoun Street, a local road, north to Illinois Route 37 and the northern terminus of Illinois Route 148 in Mount Vernon. This is a distance of 55.06 miles (88.61 km).

Illinois 142 runs northwest from Equality. Before reaching Eldorado, it intersects Illinois Route 13 and U.S. Route 45. Illinois 142 largely parallels the North Fork of the Saline River before reaching McLeansboro and intersecting Illinois Route 14 and Illinois Route 242, a spur of Illinois 142. Route 142 then crosses Interstate 64 without an interchange before entering the Mount Vernon area.

SBI Route 142 ran from Effingham to McLeansboro along what is Illinois 37 today. It was changed in the mid 1930s to its current routing. In 1947, U.S. Route 460 was applied onto Illinois 142 from Mount Vernon to McLeansboro; Illinois 142 was then moved onto what was then Illinois Route 147, which had been Illinois Route 142A, or the highway north from McLeansboro to Wayne City.

This configuration lasted until 1974, when U.S. 460 was decommissioned and the current routing for Illinois 142 re-established from Mount Vernon to McLeansboro. The road from McLeansboro north to Wayne City was again renamed, to Illinois 242.