Magnetic field sensors are widely used in various modern industrial applications, such as magnetic storage, navigation systems and position sensing. The superconducting quantum interference device (SQUID) for example is capable of detecting weak magnetic fields down to femto-tesla (fT) precision. As such, SQUIDs to date represent the highest end among the spectrum of different kinds of magnetic field sensors in terms of resolution. However, their high cost and requirement for low temperature operation conditions are some drawbacks that prevent their use in more ubiquitous applications. At the other end of the performance-price spectrum are Hall-effect sensors, which belong to a class of more mainstream magnetic field sensors. Hall-effect sensors are characterized by low cost with the tradeoff of poorer field resolution which improves at the expense of increasing their power consumption. With the development of silicon micromachining (MEMS) technology, micromechanical magnetic field sensors with comparatively lower power consumption, improved resolution and reduced barriers to CMOS integration have emerged as attractive device implementations. The design and characterization of three kinds of micromechanical magnetic field sensors will be presented in this thesis. They can be generically classified into either amplitude-based or frequency-based MEMS resonant magnetometers in terms of different sensing output. First, an amplitude-based MEMS magnetometer in the form of a horseshoe silicon micromechanical resonator is discussed. This device is actuated by a Lorentz force that causes a mechanical displacement, from which the magnetic field strength is inferred. This unique structure and vibration mode resemble a double-ended tuning fork (DETF) resonator, which leads to a higher quality factor of about 14,400 that improves the field sensitivity of the device. Next, a frequency-based resonant MEMS magnetometer shaped in the form of a DETF resonator is presented. The nominal resonant frequency of the resonator is perturbed due to the action of a Lorentz force on the resonator, from which the magnetic field strength is determined. The quality factor as high as 100,000 from the DETF anti-phase vibration mode provides for a better field resolution of the device for the same given sensitivity. Lorentz-based MEMS magnetometers all require a biasing direct or alternating current, which unavoidably introduces joule heating, which undesirably modulates the stiffness of the resonator according to the temperature coefficient of modulus of silicon. Therefore, this thesis also models the effect of joule heating on frequency tuning, which is also verified by experiments. As a step towards eliminating the need for a biasing current, this thesis also proposes an electromagnetic induction readout MEMS resonant magnetometer. By designing with compliant flexural beam resonators, improved sensitivity of 17.7 mV/T is achieved by larger displacement afforded by using beams.