US 85 in Texas begins at the Mexico–US border with US 62 and travels north through El Paso, beginning at the Santa Fe Street Bridge, and following Santa Fe Street, then Paisano Drive westward, along the Rio Grande until Paisano Drive ends where it joins with Interstate 10, about 14 miles (23 km) before both reach the New Mexico border.

This route is marked as Texas State Highway 20 north of the intersection with Mesa Street/Country Club Drive.

The unsigned route of US 85 through New Mexico exists only on paper to maintain continuity with signed sections in Colorado and Texas.

Except for a 4-mile segment through Las Vegas (signed as Business Loop 25), US 85 in New Mexico is entirely concurrent with Interstate Routes.

I-25 was also built directly over US 85 from east of Santa Fe to Las Vegas and from US 64 to the Colorado border at Raton Pass.

Consistent with this policy, NMDOT has removed US 85 from its route logs, but the American Association of State Highway and Transportation Officials (AASHTO) retains US 85 on a concurrent alignment with I-10 and I-25 to maintain continuity with signed segments in Texas and Colorado.

Many mapmakers, such as the American Automobile Association, Rand McNally and Google Maps follow AASHTO's practice and still sign US 85 along its concurrent stretches with the respective interstates.

It becomes an expressway near Chatfield Reservoir and the southern Denver suburbs of Littleton and Englewood, where it is commonly known as Santa Fe Drive.

They travel concurrently for 3 miles (4.8 km) until exit 12 when US 85 becomes an expressway and continues north out of the Denver area through Brighton.

From there it parallels I-25 for about 75 miles (121 km) passing through Fort Lupton, Platteville, LaSalle, Evans, Greeley, Eaton, Ault, Pierce, and Nunn before crossing into Wyoming.

[5] US 85 enters the Black Hills from Wyoming and travels northeast until it meets with US 14 Alternate east at Cheyenne Crossing.

The stretch from Watford City to Williston is in the process of being converted into an undivided four-lane highway, and should be substantially completed in 2014.

What is now US 85 from El Paso, Texas to then-US 66 (now NM 6) in Los Lunas, New Mexico (south of Albuquerque) was shown as U.S. Route 466 in an early 1925 plan for the U.S. Highway System.