It runs for 13.58 miles (21.85 km), entirely within Bryan County.

SH-91 has no lettered spur routes.

SH-91 begins at Denison Dam at Lake Texoma, connecting to Texas State Highway 91 and heading north.

Shortly after reaching the shore it turns east to run through Cartwright.

SH-91 was originally State Highway 75A, but was renumbered as a continuation of SH 91 in Texas.