Intraplate regions commonly host energetic earthquakes on less-prominent fault zones, raising questions on how fault structure may influence intraplate seismogenesis. Here, we investigate the causative fault of the strongly-felt April 5, 2024 Mw4.8 New Jersey Earthquake which occurred near the misoriented 300-km long Ramapo Fault. Field mapping of ancient fracture networks reveals that the up-dip projection of relocated aftershocks is co-located with a previously unmapped fault zone striking parallel to the mainshock’s nodal plane, henceforth designated as “Mountainville Fault (MF)”. MF hosts an irregular ‘stair-step’ internal structure with dominantly NNE-striking/E-dipping gougeless fracture clusters, and steeply-dipping sets cross-cut by gently-dipping slickensided surfaces, indicating an immature, rough fault. Laboratory friction experiments suggest possible shallow stress perturbations and/or concealed fault rock alteration, and with slip tendency analysis, predict the fault’s instability. These MF characteristics fit other structurally investigated earthquake sources in the region, demonstrating the seismic hazard of elusive, critically-oriented immature intraplate faults.