SR 128 begins at the three-way intersection of San Juan Avenue and Lane Avenue (SR 103), with SR 128 heading east as a four-lane street with a center turn lane through residential and commercial areas of the Hyde Grove, Hyde Park, Lake Shore, Fairfax, and Ortega neighborhoods of Jacksonville.

About one mile (1.6 km) east of its western terminus at Lane Avenue, SR 128 crosses the Cedar River in the Lake Shore neighborhood and becomes more residential than commercial east of the crossing, heading towards intersections with Cassat Avenue (SR 111), followed by Blanding Boulevard (SR 21) just two blocks to the east.

Three blocks east of Blanding Boulevard, San Juan Avenue crosses a double-tracked railway right at the western end of the intersection with US 17/SR 15 (Roosevelt Boulevard), ending the center lane for SR 128 east of the intersection, and passes by the Roosevelt Square Mall at the southeast end of the intersection.

[2] San Juan Avenue continues east of Herschel Street under the designation of SR 211 as a two-lane road, heading towards the Ortega Bridge three blocks east of SR 128's eastern terminus.

The entire route is in Jacksonville, Duval County.