The combined route is a north–south four-to-six lane suburban roadway, lying east of the Long Beach Freeway (Interstate 710) and west of the San Gabriel River Freeway (Interstate 605), and connecting the eastern parts of Long Beach and Pasadena via the Whittier Narrows.

State control of SR 19 then ends north of Gardendale Street when Lakewood Boulevard fully enters into Downey.

[7] Rosemead Boulevard continues north through Pico Rivera (where this segment of SR 19 has been relinquished to the city as well[8]) to Gallatin Road.

[9] The hidden SR 164, but signed as part of SR 19, begins north of Gallatin Road, traveling along Rosemead Boulevard across Whittier Narrows Dam before entering the Whittier Narrows Recreation Area and intersecting with State Route 60.

This portion of Rosemead Boulevard between Gallatin Road and Interstate 10 is still maintained by the state and built to expressway standards.

(The highway was relinquished to Temple City,[10] and those portions in the unincorporated areas were transferred to Los Angeles County.

Before the present SR 19 was constructed, San Gabriel Boulevard, which extended from Lamanda Park south to Downey, and was widened and repaved by an assessment tax in 1925,[12] was the main road through the Whittier Narrows, requiring several turns to stay on it and to continue south to Long Beach.

[6] 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.