We study by a fully nonlinear three-dimensional pseudospectral time-splitting simulation, the feedback control of a layer of fluid heated from below. The initial condition corresponds to a steady large-amplitude preferred convection state obtained at Prandtl number of 7.0 and Rayleigh number of 104, which is about six times the Rayleigh critical value. A robust controller based on the LQG (linear–quadratic–Gaussian) synthesis method is used. Both sensors and actuator are thermal-based, planar, andassumed to be continuously distributed. The simulated results show that large-amplitude steady-state convection rolls can be suppressed by the linear LQG controller action. The Green's function of the controller gives the shape of the control action corresponding to a point measurement. In addition, for Rayleigh numbers below the proportional feedback control stability limit, this controller appeared to be effective in damping out steady-state convection rolls as well. However, in a region very near the proportional control stability limit, proportional control action induces subcritical g-type hexagonal convection, which is obtained here through direct simulations. Note that well above this proportional control limit, the LQG still damps out all convection. The nonlinear plant model is validated by comparing check cases with published results.