When the U.S. Numbered Highway System was first defined, it was limited to Nebraska and Wyoming; by the 1950s, it continued into Idaho and Oregon.

Prior to 2004, the route's last 20 miles (32 km) were cosigned with US 101 from the highways' junction south of Seaside north to Astoria where its intersection with US 30 was also US 30's western terminus.

At its peak, immediately before the establishment of the Interstate Highway System, US 26 was 1,557 miles (2,506 km) in length and terminated in Astoria.

After passing through the Vista Ridge Tunnels, heading into Downtown Portland, it intersects with I-405 and runs along the Interstate southbound for about a half-mile (0.80 km) before exiting onto surface streets at the waterfront, meeting Oregon Route 43 (OR 43) at Macadam Avenue before crossing I-5.

26, a four-lane divided highway that was supposed to be the Mount Hood Freeway, which was never built just south of Division Street.

The Mount Hood Highway continues north along OR 35, while US 26 heads southeast toward Madras, where it intersects with US 97.

From the Oregon state line, US 26 continues to follow US 20 to Boise, with short concurrencies with US 95 near Parma and I-84 at Caldwell.

US 189 ends in Jackson, and the other three highways continue their concurrency through Grand Teton National Park up to Moran.