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). This route was transferred to
FM 84 on October 26, 1943, when portions of SH 91 were submerged in the rising
Lake Texoma. The current routing of SH 91 was originally built on May 18, 1944, as
State Spur 151, connecting the newly completed Denison Dam along the Red River to the city of Denison. The route was renumbered on January 26, 1946, to State Highway 75A. This was to correlate with the similarly numbered Oklahoma Highway 75A that it connected to at the Red River. Both of these routes were numbered 75A to provide an alternate route for US Route 75, which crossed the Red River about 2 miles downstream from the dam. 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. The route was extended a final time in 1994 when US 75 was again rerouted around Denison, this time as a limited access highway to the west of town on March 29, 1957. On September 23, 1959, the bridge to Oklahoma was added to the designation. The route was extended south along Business US 75 to the north side of
Sherman on December 21, 1994. State Highway 75A was also renumbered to State Highway 91 at this time to prevent confusion with the main US Route and was again coordinated with a renumbering of the adjacent
Oklahoma State Highway 91. ==Route description==