Iowa Highway 187 begins at an intersection with US 20 southwest of Masonville. It heads in a northward alignment from this point, passing through Lamont. North of Lamont, it intersects with IA 3 before turning to the northwest. The highway passes through Arlington before turning directly to the west. Further west, it will turn to the north where it will terminate at an intersection with IA 150 south of Fayette.
At the time of designation, the highway was originally aligned from US 20 heading northward to the western gate of the Backbone State Park, replacing the old IA 208. In 1980, the alignment had a major overhaul, being realigned northward through Lamont and ending at IA 3. Then in 1981, it was extended again to IA 150 south of Fayette, which is its present day designation.