The focus of this paper is to develop and improve a higher-order Haar wavelet approach for solving nonlinear singularly perturbed differential equations with various pairs of boundary conditions like initial, boundary, two points, integral and multi-point integral boundary conditions. The theoretical convergence and computational stability of the method is also presented. The comparison of the proposed higher-order Haar wavelet method is performed with the recent published work including the well-known Haar wavelet method in terms of convergence and accuracy. In the nonlinear case, a quasilinearization technique has been adopted. The proposed method is easy to implement on various boundary conditions, and the computed results are high-order accurate, stable and efficient. We have also checked the satisfactory performance of the proposed method for nonlinear differential equations having no analytical solution in some of the test problems.