Typically, unit loads with wooden pallets in warehouses are moved by pallet lifters or AMRs which are bulkier in size and require more power with high operating costs. This disclosure relates generally to a unit load lifter designed with counterbalance arm mounted on an autonomous mobile robot (AMR) to load and unload unit load from one position to another position autonomously. The unit load lifter includes a horizontal slide unit and a vertical axis fork assembly. The horizontal slide unit include base plate of the unit load lifter is mounted on the AMR. A plurality of fixed guides is integrated with the base plate to house the vertical axis fork assembly by a plurality of rollers on a roller mounting plate. The vertical axis fork assembly include an actuating end of a linear actuator is connected to the sliding plate to drive the at least one fork up and down.