A system for mounting a headrail for a covering for an architectural opening on a mounting bracket comprising in combination: a headrail having a support arm and a wall spaced therefrom to define a pocket therebetween, and a mounting bracket including: (a) means for connecting the mounting bracket to a frame of an architectural opening; (b) a base adapted to be inserted into said pocket; (c) a depressible catch arm on said base movable from a neutral position operatively engageable with said wall to releasably secure said base in said pocket; (d) a lock bar mounted on said base in operative relationship with said catch arm, said lock bar being slidably movable between locking and release positions, said lock bar engaging said support arm in said locking position to prevent depression of said catch arm to positively secure said head rail to said mounting bracket; (e) a lock lever operatively associated with said lock bar for moving said lock bar between said locking and release positions; and characterised in that the system further comprises: (f) a resilient member releasably biasing said lock bar in said locking and release positions; and (g) wherein said lock lever includes a boss engageable with said wall when said head rail is being mounted on said bracket, the engagement of said wall with said boss permitting said wall to automatically pivot said lock lever from said release position to said locking position. a headrail having a support arm and a wall spaced therefrom to define a pocket therebetween, and a mounting bracket including: (a) means for connecting the mounting bracket to a frame of an architectural opening; (b) a base adapted to be inserted into said pocket; (c) a depressible catch arm on said base movable from a neutral position operatively engageable with said wall to releasably secure said base in said pocket; (d) a lock bar mounted on said base in operative relationship with said catch arm, said lock bar being slidably movable between locking and release positions, said lock bar engaging said support arm in said locking position to prevent depression of said catch arm to positively secure said head rail to said mounting bracket; (e) a lock lever operatively associated with said lock bar for moving said lock bar between said locking and release positions; and characterised in that the system further comprises: (f) a resilient member releasably biasing said lock bar in said locking and release positions; and (g) wherein said lock lever includes a boss engageable with said wall when said head rail is being mounted on said bracket, the engagement of said wall with said boss permitting said wall to automatically pivot said lock lever from said release position to said locking position. (a) means for connecting the mounting bracket to a frame of an architectural opening; (b) a base adapted to be inserted into said pocket; (c) a depressible catch arm on said base movable from a neutral position operatively engageable with said wall to releasably secure said base in said pocket; (d) a lock bar mounted on said base in operative relationship with said catch arm, said lock bar being slidably movable between locking and release positions, said lock bar engaging said support arm in said locking position to prevent depression of said catch arm to positively secure said head rail to said mounting bracket; (e) a lock lever operatively associated with said lock bar for moving said lock bar between said locking and release positions; and characterised in that the system further comprises: (f) a resilient member releasably biasing said lock bar in said locking and release positions; and (g) wherein said lock lever includes a boss engageable with said wall when said head rail is being mounted on said bracket, the engagement of said wall with said boss permitting said wall to automatically pivot said lock lever from said release position to said locking position.