We report a highly sensitive and miniature in-plane resonant Lorentz-force magnetic micro-sensor operating in air at atmospheric pressure. The concept is based on the detection of the resonance frequency shift of an electrothermally heated initially curved micro-beam experiencing the veering phenomenon (avoided crossing) between its first and third vibration modes. Finite element method (FEM) and experimental results show that the proposed micro-sensor exhibits high sensitivity around the veering regime for the third mode. When operated in the first mode, the micro-sensor shows a measured sensitivity (S) of 0.16/T, which is very high compared to the state of the art. At the veering phenomenon, the third mode is very sensitive to perturbations, and hence the micro-sensor becomes even more ultra-sensitive (S=0.32/T) making it promising for various magnetic field applications. We report a minimum detectable magnetic field of 20 mT at atmospheric pressure.