An apparatus for facilitating the rapid mounting of fence boards to fence posts includes a rail having a tongue projecting perpendicularly from one end thereof; and a board support assembly slidably disposed on the rail, the board support assembly including a carriage slidably mounted on the rail, a carriage lock for selectively fixing the carriage at a desired location along the rail, and a hook depending from the carriage below the tongue for temporarily supporting one end of a board to be mounted to a post. The subject apparatus may be hung from a first fence board by the tongue and then used to support an end of a second fence board within the cradle a desired distance below, and in substantially coplanar alignment with, the first board for mounting to a post.