An experimental and numerical investigation of mixed convection phenomena about a finite-length, vertical, cylindrical heat source in a uniform, liquid-saturated, porous medium was conducted. Buoyancy-induced upflow about the heat source was systematically altered by the superposition of vertical, pressure-driven flows which opposed the buoyancy-induced fluid motion. The evolution of the mixed convection velocity and thermal fields with increasing magnitude of the imposed-flow Peclet number are reported. The ratio of the natural convection Rayleigh number Ra to the imposed-flow Peclet number Pe is shown to be the nondimensional parameter that characterizes the relative influence of buoyancy-induced to pressure-driven fluid motion. Using total disappearance of buoyancy-induced upflow as the criterion, the transition from mixed to forced convection, for opposing flows, is numerically predicted to occur for |Ra/Pe| ≈ 1/2, independent of the heat source length or power input.