We propose a simple phased-array design based on a Fabry-Perot cavity antenna for the generation of a highly-directive pencil beam steerable along two directions in 3-D space. Each array element generates an element pattern in the far-field obtained through the excitation of a dominant cylindrical TM leaky wave inside the cavity. Hence, the resulting conical pattern is combined with the array factor of a N × N square or circular arrangement of ideal vertical electric dipoles. By proper phasing such sources at the operating frequency, a pencil beam with continuous scanning both in azimuth and elevation is achieved. The proposed leaky-wave phased array is of interest for future wireless power transfer systems as well as for advanced radar and localization systems.