A structure and method for mounting a flag-type side mirror securely fastens the flag-type side mirror on a door panel of a vehicle. The structure includes: a mirror neck (11) of the flag-type side mirror which is fastened to the door (20) formed with a plurality of panels bonded to each other, where the mirror neck (11) has an end fastened to one panel of the plurality of panels of the door (20), and a portion of the panel to which the mirror neck (11) is fastened is circumferentially bonded to another panel of the plurality of panels of the door (20).