Pulsed laser welding of ultra-thin Mg–Li alloy LZ91 with a thickness of 1 mm was conducted to investigate the effects of laser power, duty cycle, and frequency on weld formation, microstructure, and mechanical properties. The results show that fully penetrated welds with good forming consistency can be obtained by pulsed laser welding. The microstructure of the LZ91 welded joint heat-affected zone is coarse α-Mg and β-Li dual phase structure, and numerous short acicular α-Mg phases are distributed in β-Li grains of the weld zone. The increase of laser power has little effect on the phase composition, but leads to an increase in β grain size in both the heat-affected zone and weld zone. The elongation and impact absorbing energy of welded joints decrease with the increase of laser power, whereas the overall changes in tensile strength and hardness are relatively small. The microhardness of the weld zone is the highest, followed by the base metal, while the heat-affected zone has the lowest microhardness.