Subsynchronous control interaction (SSCI) is a fast growing electrical interaction between series-compensated transmission lines and power electronic converters of DFIGs that may destroy the machines if not prevented. Thus, a nonlinear controller is proposed in order to mitigate SSCI in series-compensated doubly-fed induction generator (DFIG)-based wind farms. Partial feedback linearization technique is utilized in developing the controller that involves a number of design steps. To evaluate the performance of the proposed controller, eigenvalue (modal) analysis and electromagnetic transient (EMT) simulation are performed on a 100 MW DFIG-based offshore wind farm connected to the grid through series-compensated lines. Eigenvalue study reveals the presence of several unstable subsynchronous modes with the PI controller while the system mostly retains stability with the use of the designed Partial feedback linearizing (PFL) controller unless the compensation level is too high (above 90%). EMT simulation, on the other hand, shows that the prevalent SSCI in the system with the PI controller is effectively damped through the use of the designed PFL controller in less than 2.3 s.