Steady mixed convection boundary layer flow from an isothermal horizontal circular cylinder embedded in a porous medium filled with a nanofluid has been studied for both cases of a heated and cooled cylinder. The resulting system of nonlinear partial differential equations is solved numerically using an implicit finite-difference scheme. The solutions for the flow and heat transfer characteristics are evaluated numerically for various values of the governing parameters, namely the nanoparticle volume fraction phi and the mixed convection parameter lambda. Three different types of nanoparticles are considered, namely Cu, Al2O3 and TiO2. It is found that for each particular nanoparticle, as the nanoparticle volume fraction phi increases, the magnitude of the skin friction coefficient decreases, and this leads to an increase in the value of the mixed convection parameter lambda which first produces no separation. On the other hand, it is also found that of all the three types of nanoparticles considered, for any fixed values of phi and lambda, the nanoparticle Cu gives the largest values of the skin friction coefficient followed by TiO2 and Al2O3. Finally, it is worth mentioning that heating the cylinder (lambda > 0) delays separation of the boundary layer and if the cylinder is hot enough (large values of lambda > 0), then it is suppressed completely. On the other hand, cooling the cylinder (lambda < 0) brings the boundary layer separation point nearer to the lower stagnation point and for a sufficiently cold cylinder (large values of lambda < 0) there will not be a boundary layer on the cylinder.