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We present new Goddard High Resolution Spectrograph measurements of the Milky Way halo gas absorption toward Markarian 509 in the 1233-1268 {\AA} and 1521-1555 {\AA} spectral regions. We detect high velocity absorption in both members of the C IV doublet but find no corresponding Si II or N V at LSR velocities of -340 to -180 km/sec. The detection of strong C IV combined with the observed limits on N V, Si II, and H I 21 cm emission measured with the NRAO 140 foot telescope strongly suggest that the high velocity C IV arises at large distances from the Galactic plane in gas ionized by extragalactic background radiation. If true, this detection represents one of the few cases where halo gas absorption at large distances is cleanly separated from absorption by disk gas at lower velocities. Mapping of the 21 cm emission in the area around Markarian 509 indicates that the high velocity C IV absorption probably occurs at the boundary of one or more high velocity H I clouds located in the same general direction. The high velocity C IV absorption has properties more closely resembling absorption associated with the majority of quasar metal line absorption systems than absorption along previously studied sight lines through the Milky Way disk and halo.
