Using recently published global magnetic models, we present the first independent constraint on North Atlantic geothermal state and mantle dynamics from magnetic anomaly inversion with a fractal magnetization model. Two theoretical models of radial amplitude spectrum of magnetic anomalies are found almost identical, and both are applicable to detecting Curie depths in using the centroid method based on spectral linearization at certain wave number bands. Theoretical and numerical studies confirm the robustness of this inversion scheme. A fractal exponent of 3.0 in the magnetic susceptibility is found suitable, and Curie depths are well constrained by their known depths near the mid-Atlantic ridge. While generally increasing with growing ages, North Atlantic Curie depths show large oscillating and heterogeneous patterns related most likely to small-scale sublithospheric convections, which are found to have an onset time around 40 Ma and a scale of about 500 km, and are in preferred transverse rolls. Hotspots in North Atlantic also contribute to large geothermal and Curie-depth variations, but they appear to connect more closely to geochemical anomalies or small-scale convection than to mantle plumes. Curie depths can be correlated to heat flow gridded in a constant 1° interval, which reveals decreasing effective thermal conductivity with depths within the magnetic layer. North Atlantic Curie points are mostly beneath the Moho, suggesting that the uppermost mantle is magnetized from serpentinization and induces long-wavelength magnetic anomalies. Small-scale convection and serpentinization together may cause apparent flattening and deviations in heat flow and bathymetry from theoretical cooling models in old oceanic lithosphere.