[1] The route serves downtown San Diego, Mission Valley, and the cities of La Mesa and El Cajon.

It traverses Mission Valley and the San Diego River corridor for this segment.

During a system redesign which took effect on September 2, 2012, as part of the Trolley Renewal Project, the western portion of the Green Line was extended from Old Town south through downtown and along the Bayside Line, terminating at 12th & Imperial Transit Center's Bayside Platform.

[10] In May 2024, MTS announced that a new service, known as the East County Connector (and later the Copper Line), would replace the Green and Orange lines between El Cajon Transit Center and the eastern terminus at Santee.

[11] The Green Line's SDSU Transit Center is the only underground station in the entire trolley network.