It generally runs northeast from downtown Modesto near the SR 99/SR 132 interchange, to U.S. Route 395 near the Nevada state line.

The route was once recommended to continue south of Modesto to Interstate 5, although today that portion exists as a county road.

After several blocks eastward on Needham Street, SR 108 reaches McHenry Avenue, which it follows north out of the city.

From Modesto, SR 108 runs north along McHenry Avenue for about 7 miles (11 km) before turning eastward on Patterson Road to follow the Stanislaus River east-northeast.

SR 108 continues eastward and gains elevation as it passes through Twain Harte, Mi-Wuk Village, Long Barn, Cold Springs, Strawberry-Pinecrest Lake and Dodge Ridge Ski Area, then on to Dardanelle and Kennedy Meadow.

At the pass, the highway crosses the summit of the Sierra Nevada and enters Mono County, then descends past the Mountain Warfare Training Center at Pickel Meadows, finally reaching the eastern terminus at Sonora Junction where it meets U.S. Route 395.

[7] Except where prefixed with a letter, postmiles were measured on the road as it was in 1964, based on the alignment that existed at the time, and do not necessarily reflect current mileage.