A frequency-agile mm-Wave power amplifier capable of reconfiguring itself to operate near-optimally over a wide range of tunable frequencies, yet producing output power >22dBm with PAE>20%, is useful for a wide range of applications in reconfigurable communication systems supporting multi-Gb/s over large links to hyper-spectral sensing and imaging. While recent works have demonstrated high efficiency in Watt-level mm-Wave power amplifiers [1-5], most of the architectures operate over a limited frequency range, primarily dominated by the bandwidth of the output-matching or combiner networks. A frequency-agile power amplifier, therefore, needs a widely reconfigurable, yet an efficient output network, which is difficult to achieve with on-chip variable passives due to their poor quality factors in silicon IC processes. In this paper, we present an architecture where an optimal impedance at the output of each unit cell is synthesized exploiting their interaction in a non-isolated power-combiner network. By controlling the amplitude and phase of unit cells, close-to-optimal impedances can be synthesized over a wide operating range at mm-Wave frequencies thus enabling frequency agility. The chip is implemented in a 0.13μm SiGe BiCMOS process and achieves Psat>22dBm between 40 and 65GHz and peak PAE>25% between 40 and 60GHz.