Most of the existing results on anti-disturbance consensus control of multi-agent systems focus on matched-disturbance rejection and the disturbances are assumed to be constant or slowly time-varying. This paper investigates the consensus control problem of double-integrator multi-agent systems with mismatched disturbances, which can be some kinds of faster time-varying disturbances, such as ramp and higher-order disturbances. To estimate the higher-order disturbances and their derivatives, for each agent, a generalized proportional integral observer (GPIO) is constructed. By distributedly employing the disturbances estimates, a kind of nonlinear sliding-mode surfaces are developed for both leaderless and leader-follower multi-agent systems. Based on the proposed surfaces and the disturbances estimates, composite consensus protocols are designed for both cases, which guarantee the global asymptotical stability of the consensus error systems. Simulation results show the effectiveness of the proposed algorithms.