We report on the first implementation of phase apodization for high-contrast imaging at close inner working angle. It is designed for use in the 5 μm M band with the adaptive optics system at the MMT, which uses a deformable secondary for low thermal background and achieves a Strehl ratio of 90% at 5 μm. The method uses a diamond-turned ZnSe phase plate located at a cold pupil stop to diffract starlight into an "anti-halo" which suppresses the Airy diffraction pattern over a semi-circular region around the star. The design was optimized for strong suppression from the first bright Airy ring out to the control radius achievable with the MMT deformable secondary, about 9λ/D. The time-averaged PSF of a bright star agrees well with the design profile, the core with FWHM of 0.18 arcsec showing the diffraction-limited resolution of the full aperture. At 0.34 arcsec radius (2λ/D) the floor level is 3.5 × 10 -3 of the central peak, limited by residual atmospheric errors with 3 m wavelength across the aperture. The measured fluctuations at this radius averaged over 20 seconds are 2.5 × 10 -4 rms (9 magnitudes down from the peak). With the addition of active feedback to control residual speckles caused by static wavefront errors and with longer exposures, we project that exoplanet searches should reach 5σ sensitivity level > 10 magnitudes in an hour of integration.