The original SH 91 was designated on October 15, 1923 on a similar route from Denison north to a different Red River crossing west of the current one, where it connected with Oklahoma Highway 48 (now OK-70A).

[2] This route was transferred to FM 84 on October 26, 1943 when portions of SH 91 were submerged in the rising Lake Texoma.

SH 75A extended south from FM 84 (former SH 91) to the new location of US 75 The route originally ended on the northwest side of Denison connecting with the original route of US 75, but was extended southward through the residential sections of town to the new intersection with US 75 when it was rerouted along the eastern side of town.

State Highway 91 begins at an intersection with US Route 75, exit 61, on the north side of Sherman.

The route then continues northward through residential sections of western Denison before exiting town to the northwest.