Self-assembled heteroepitaxial bismuth ferrite - cobalt ferrite (BFO-CFO) nanocomposite films were grown on (001) oriented piezoelectric Pb(Mg1/3Nb2/3)(0.72)Ti0.28O3(001) (PMN-PT0.28) substrates by pulsed laser deposition in which BFO and CFO targets were alternately ablated. Completely reversible electric field induced biaxial strain in this substrate has been exploited to produce reversible electric field control of magnetization in the BFO-CFO nanocomposite film. The surface of the CFO-BFO films shows rectangular CFO pillars protruding out of a flat BFO matrix. The pillars are aligned with their edges along the pseudocubic {110} directions and are homogeneously distributed in the matrix. X-ray diffraction revealed an out-of-plane compression of the CFO unit cell by similar to 0.4%. Magnetization hysteresis (M-H) loops show a moderate perpendicular anisotropy of the magnetostrictive CFO pillars, which is related to their vertical compression. The application of an electric field to the electromechanical PMN-PT0.28 substrate produced significant and reversible changes in the magnetization due to an additional strain-induced magnetic anisotropy. This demonstrates completely reversible, room-temperature electric-field-assisted control of magnetization in self-assembled vertical nanocomposites of CFO and BFO.