Achieving strength–ductility synergy in immiscible Ti/Mg laminated composites remains challenging, owing to the absence of metallurgical bonding and the limited slip systems of their hexagonal close-packed (HCP) structures. This study aims to achieve a favorable strength–ductility balance in immiscible Ti/Mg laminated composite plates via low-temperature annealing after hot-roll bonding. The results show that after post-rolling annealing at 200 °C, the bonding strength of the Ti/Mg composite interface increased from 48.74 MPa to over 80 MPa without the formation of obvious intermetallic compounds. In the Mg layer, complete recrystallization eliminated the shear bands, and pyramidal slip—exhibiting the highest Schmid factor—became the dominant strain-accommodation mode. In the Ti layer, recovery, rather than full recrystallization, prevailed. Concurrently, basal and pyramidal slip were co-activated to enable multi-slip load bearing. These complementary deformation modes, coordinated through the strengthened interface, promoted compatible deformation between the layers, in which the plastic flow of the Mg layer relieved local strain concentration in the Ti layer, resulting in a more than twofold increase in elongation with minimal loss of strength. Annealing at 200 °C was identified as the preferred annealing temperature, offering a viable pathway to strength–ductility synergy in immiscible lightweight Ti/Mg laminated composite plates.