SR 91 begins at the Tennessee state line southeast of Damascus. The highway continues south into Tennessee as
SR 91 toward
Mountain City. SR 91 heads northwest as Mountain City Road, which parallels Laurel Creek through the
Iron Mountains to the creek's confluence with Whitetop Laurel Creek. Just west of that confluence, the state highway crosses Laurel Creek, intersects the
Virginia Creeper Trail rail trail, and
runs concurrently with US 58 (Jeb Stuart Highway) parallel to the creek and trail into the town of Damascus. When US 58 turns west to cross Laurel Creek into the center of town, SR 91 continues north and then west on Damascus Drive. The state highway veers north away from Laurel Creek and leaves the town as Monroe Road, which passes through the village of
Lodi between crossings of the South Fork and Middle Fork of the
Holston River. Just south of Glade Spring, SR 91 intersects US 11 (Lee Highway) and continues north as Maple Street through a
diamond interchange with I-81 and into the town. When Maple Street continues straight as
SR 91 Business, SR 91 veers east as Monte Vista Drive to bypass the downtown area. The state highway receives the northern end of the business route, Glade Street, east of downtown. SR 91 crosses over
Norfolk Southern Railway's
Pulaski District and exits the town on Crescent Road, which passes through McCall Gap in Walker Mountain. Shortly after entering the town of Saltville, where the highway is named Main Street, the highway veers east and crosses the Washington–Smyth county line. At the east end of the downtown area, SR 91 meets the northern end of
SR 107 (Worthy Boulevard). The state highway crosses the North Fork Holston River and parallels it east as Saltville Highway to the hamlet of
Broadford, where the highway meets the southern end of
SR 42 (Blue Grass Trail) and turns north to follow Laurel Creek through Brushy Mountain. SR 91 continues northeast as Veterans Road into Tazewell County. The state highway briefly follows Laurel Creek through the Freestone Valley before ascending Flattop Mountain near
Tannersville. At the end of its descent from the mountain, SR 91 meets the western end of SR 607 (Little Tumbling Creek Road) and crosses the eponymous creek. Just north of the bridge, the state highway's surface changes from asphalt to gravel. The gravel road continues for about on the state highway's crossing of
Clinch Mountain. SR 91 returns to asphalt road just south of its intersection with SR 604 (Thompson Valley Road), from which the highway follows Maiden Spring Road. At the hamlet of Maiden Spring, the state highway crosses the
Little River, a tributary of the
Clinch River, and veers east to the hamlet of
Liberty. SR 91 continues east as Witten Valley Road through the namesake valley to its northern terminus at US 19 Business and US 460 Business (Crab Orchard Road) at Frog Level. ==Major intersections==