The present disclosure is directed to a pre-formed, continuous structural component for use in assembling a modular rotor blade for a wind turbine. Further, the structural component provides support to the modular rotor blade during operation. The pre-formed structural component includes a root portion and a body portion. The root portion is configured for mounting the structural component to a blade root section of the rotor blade. The body portion is configured to extend in a generally span-wise direction. Further, the body portion defines a predetermined cross-section having a flatback portion with a first end and a second end. In addition, the first and second ends each have a flange extending perpendicularly therefrom. Thus, each flange defines a mounting surface for one or more blade segments.