We present measurements of the interferometrically resolved binary star system 12Com and the single giant star 31 Com in the cluster Coma Berenices. 12Com is a double-lined spectroscopic binary system consisting of a G7 giant and an A3 dwarf at the cluster turnoff. Using an extensive radial velocity data set and interferometric measurements from the Palomar Testbed Interferometer and the Center for High Angular Resolution Astronomy array, we measured masses M1=2.64{+/-}0.07M{sun} and M2=2.10{+/-}0.03M{sun}. Interferometry also allows us to resolve the giant and measure its size as R1=9.12{+/-}0.12{+/-}0.01R{sun}. With the measured masses and radii, we find an age of 533{+/-}41{+/-}42Myr. For comparison, we measure the radius of 31Com to be 8.36{+/-}0.15R{sun}. Based on the photometry and radius measurements, 12ComA is likely the most evolved bright star in the cluster, large enough to be in the red giant phase, but too small to have core helium burning. Simultaneous knowledge of 12ComA's mass and photometry puts strong constraints on convective core overshooting during the main-sequence phase, which in turn reduces systematic uncertainties in the age. Increased precision in measuring this system also improves our knowledge of the progenitor of the cluster white dwarf WD1216+260.