Increasing the efficiency of wind farms is important for speeding up the transition from fossil fuels to renewable energy sources. Current wind farm control relies on maximization of power generation of individual turbines. However, research has demonstrated that plant-wide wind farm control could optimize the performance of a wind farm. Wind farm simulation tools are crucial in designing, testing, and validating wind farm controllers. Fatigue, Aerodynamics, Structures, and Turbulence Farm tool (FAST.Farm) is a recently developed multi-physics engineering tool for modeling power performance and structural loads by solving the aero-hydro-servoelastic dynamics of each individual turbine within a farm. FAST.Farm aims to balance the need for accurate modeling of the relevant physics while maintaining low computational costs. However, designing controllers in FAST.Farm lacks flexibility and interactivity compared with MATLAB/Simulink. The capabilities of FAST.Farm for control design purposes can be extended through a co-simulation with MATLAB/Simulink. Therefore this thesis presents a FAST.Farm and MATLAB/Simulink interface. Consequently, this interface was used to implement and simulate wind farm controllers in FAST.Farm. FAST.Farm and MATLAB/Simulink are coupled by linking the individual Open Fatigue, Aerodynamics, Structures, and Turbulence tool (OpenFAST) instances in FAST.Farm to MATLAB with the use of an Message Passing Interface (MPI) and MATLAB Executable (MEX) functions. An Active Power Controller (APC) was implemented and simulated in FAST.Farm with the use of this interface. The APC responds to grid requirements through the control of wind farm power output. Comparing FAST.Farm and Simulator fOr Wind Farm Applications (SOWFA) simulation results of the APC shows that FAST.Farm reduces the computation time for a 10-minute simulation from 24 hours to 15 minutes, with little detriment to the accuracy of the simulation results. Although SOWFA remains the preferred validation tool, the FAST.Farm and MATLAB/Simulink interface supports developing and accelerates testing advanced closed-loop control at the wind turbine and wind farm levels.