Observations of the mercury-manganese star phi Her with the Navy Prototype Optical Interferometer (NPOI) conclusively reveal the previously unseen companion in this single- lined binary system. The NPOI data were used to predict a spectral type of A8 V for the secondary star phi Her B. This prediction was subsequently confirmed by spectroscopic observations obtained at the Dominion Astrophysical Observatory. phi Her B is rotating at 50 +/- 3 kms s(-1), in contrast to the 8 km s(-1) lines of phi HerA. Recognizing the lines from the secondary permits one to separate them from those of the primary. The abundance analysis of phi Her A shows an abundance pattern similar to those of other HgMn stars, with Al being very underabundant and Sc, Cr, Mn, Zn, Ga, Sr, Y, Zr, Ba, Ce, and Hg being very overabundant.