FM 109 begins at an intersection with Business SH 71 about 0.5 miles (0.8 km) northwest of the bridge over the Colorado River.

The highway gradually swings to the east-northeast for 3.8 miles (6.1 km) before entering New Ulm in Austin County.

At New Ulm, the highway crosses the BNSF Railway tracks and intersects with FM 1094 from the east.

Just inside the southern city limits of Brenham, FM 109 crosses the BNSF Railway tracks and ends at a traffic signal on SH 36.

[2] FM 109 was originally designated on August 1, 1944, to go from Industry to New Ulm and from Frelsburg to SH 71 near Columbus.