A memory effect has been demonstrated in magnetic nanowire arrays. The magnetic nanowire array has the ability to record the maximum magnetic field that the array has been exposed to after the field has been turned off. The origin of the memory effect is the strong magnetic dipole interaction among the nanowires. Switching field distributions among nanowires has been studied with a first-order reversal curve technique to elucidate the discrepancy between the experimental result and the theoretical explanation. Based on the memory effect, a novel and extremely low cost EMP detection scheme is proposed.