Thermal plumes have been widely investigated in a variety of scenarios, including natural convection and stratified environments. The resulting theory may be used to predict ventilation flow rates in, for example, natural and displacement ventilation, and under-floor air distribution (UFAD) systems. However, there has been little effort in investigating how uniform upward flows affect the plume velocity, rate of growth, and thermal profile. Such situations can arise if, for example, the diffusers of a UFAD system are evenly distributed. In order to study such situations, analytical expressions for the velocity and temperature profiles of a plume under mixed convection will be presented and compared with experimental results. The new model performs as well as the standard plume model, and also explains some deviations between observations and the standard model.