In this paper, an approximate analytical solution is obtained for the unsteady mixed convection flow near the stagnation region of a heated vertical plate. The unsteadiness in the flow field is caused by impulsively creating motion in the free stream and at the same time suddenly raising the surface heat flux above its surroundings. This study gains importance when the buoyancy forces due to the temperature difference between the surface and the free stream become large. The Homotopy Analysis Method (HAM) is applied to solve the coupled system of non linear partial differential equations for analytical solutions. The numerical results of the flow are computed using the Keller-Box Method (KBM). A detailed error analysis is performed to compute the total average squared residual errors for velocity and temperature. It is shown that a more accurate solution can be obtained with least computational effort by the computed approximate analytical series solutionof velocity and temperature.