Narrow backset auto-latching mortise lock for sliding door is a mortise lock that is fully contained within a 1.5 inch wide casing or housing that includes actuated automatic latching and automatic unlatching mechanism. Narrow backset auto-latching mortise lock for sliding door is a mortise lock that properly fits a narrow backset sliding door with a backset dimension of 1.25 inches. The design of narrow backset auto-latching mortise lock for sliding door has been elongated so that the latch or bolt is moved downwards and positioned below the lever hub. The positioning of the latch or bolt at a location below the lever hub requires special linkage and mechanical components in order to make the mortise lock function properly. This design required all new lock components, including a new latch lever with a dual hub arm, a dual latch arm, and a dual latch arm pin.