This work proposes a new class of controllers for mobile manipulators subject to both undesirable forces exerted on the end-effector and slip reaction forces acting on the platform wheels, unknown friction forces coming from joints directly driven by the actuators as well as undesirable forces caused by kinematic singular configurations appearing on the mechanism trajectory. Based on suitably defined task space non-singular terminal sliding manifold (TSM) and the Lyapunov stability theory, we derive a class of estimated extended transposed Jacobian controllers which seem to be effective in counteracting the unstructured forces. Moreover, in order to eliminate (or to alleviate) undesirable chattering effects, the proposed control law involves second order sliding technique. The numerical computations closely related to an experiment, which are carried out for a mobile manipulator consisting of a platform of (2, 0) type and holonomic manipulator of two revolute kinematic pairs, illustrate the performance of the proposed controllers.