Quantum sensing is crucial for precision measurements, yet quantum sensor sensitivity is often limited by the coherence time of the quantum system. Here, we demonstrate a method to enhance coherence time through cooperative spins. Using a tunable feedback circuit, we induce cooperation among noble-gas ^{129}Xe spins, resulting in an impressive 18-fold coherence enhancement. Moreover, we show that the cooperative ^{129}Xe spins can significantly amplify magnetic signals by at least 3 orders of magnitude. Magnetic field sensing assisted with such a cooperative spin amplifier realizes the sensitivity of 4 fT/Hz^{1/2} and surpasses the spin-projection noise of the embedded ^{87}Rb spin gas magnetometer. These results pave the way for a new class of "cooperative quantum sensors," and open up exciting prospects in fundamental physics.