A guard and associated method for rain gutters includes a cap or deflector extending over a gutter of the type having a trough-shaped interior, a rear wall extending along a roof fascia, and a front lip with a channel and an end flange. A mounting bracket has a rear portion abutting the rear wall of the gutter, an upper portion supporting the deflector, and a front portion with a hook-shaped nose. The mounting bracket nose is configured for insertion under the end flange of the gutter, such that the mounting bracket is rotated rearwardly along a generally vertical arc about the nose toward the rear wall of the gutter, and shifted laterally into a skewed orientation within the gutter interior. The mounting bracket is then pivoted laterally along a generally horizontal plane about the nose into a perpendicular orientation within the gutter interior to retain the nose in the front lip of the gutter, and facilitate attachment of the rear portion of the mounting bracket to the rear wall of the gutter and the roof fascia.