A leg rest assembly includes two connecting boards fixed to an underside of a seat of a chair. An actuation rod unit and a link unit are mounted to each connecting board. A rear end of each link unit is pivotably connected to one of the connecting boards and one of the actuation rod units. Each link unit has a slot aligned with a guiding groove of one of the connecting boards. A guiding rod extends through the slot of each link unit, the guiding groove of each connecting board, and each actuation rod unit. A mounting plate is pivotably connected to a front end of each link unit. An operative rod is operable to cause movement of the actuation rod units and to move the mounting plate between a first position in front of the base and a second position received in a compartment between the connecting boards.