[1][2][3] SH 186 begins at an intersection with US 281 (future I-69C) and FM 1917 in Linn (a census-designated place formerly known as San Manual-Linn) in Hidalgo County.

)[2] From its western terminus SH 186 travels to the southeast, passing a section of the Lower Rio Grande Valley National Wildlife Refuge, before turning to a more easterly path and crossing into Willacy County.

SH 186 runs along Hidalgo Avenue in Raymondville, where it crosses Interstate 69E / U.S. Route 77.

In Port Mansfield, SH 186 intersects the short Farm to Market Road 606 near its eastern terminus; the highway continues one block past this point before ending at Laguna Madre.

SH 186 was extended to its current eastern terminus in Port Mansfield on August 29, 1990, replacing FM 497.