This study presents a theoretical analysis for the problem of steady, laminar mixed convection heat transfer from a hot/cold vertical plate embedded in a fluid-saturated porous medium. The effect of viscous dissipation is included. The governing systems of partial differential equations are solved using a suitable variable transformation and employing the fourth-order Runge-Kutta integration and the shooting method. The governing parameters are, namely, the mixed convection parameter, the Eckert number, the porosity and non-Darcian effects, and the Prandtl number. A parametric study is conducted in order to study the interactive effects of these parameters on the velocity and temperature profiles as well as on the Nusselt number. Based on the facts that the free stream is either in parallel or reverse to the gravity direction and the plate is heated or cooled, four different flow situations are identified: aiding (where the buoyancy force accelerates the flow) with positive Ec (wall heating), aiding with negative Ec (wall cooling), opposing (where the buoyancy retards the flow) with positive Ec, and opposing with negative Ec. Very interesting thermophysical behaviors are observed and discussed for each case. The obtained results are checked against previously published work on special cases of the problem and found to be in excellent agreement.