We construct the first explicit non-trivial example of deformed Hermitian Yang-Mills (dHYM) instanton on a higher rank slope-unstable holomorphic vector bundle over a Fano threefold. Additionally, we provide a sufficient algebraic condition in terms of central charges for the existence of dHYM instantons on Whitney sum of holomorphic line bundles over rational homogeneous varieties. As a consequence, we obtain several new examples of dHYM instantons on higher rank holomorphic vector bundles.