This dissertation presents the design, fabrication, and evaluation of the first monolithically integrated MEMS resonant sensor system realized in the InP-InGaAs material family. The integration of a MEMS sensor along with the facilitating optical interrogation platform provides for increased manufacturing scalability, sensitivity, and reduced measurement noise and device cost. The MEMS device presented in this dissertation consists of an Indium Phosphide (InP) cantilever waveguide resonator whose displacement is measured optically via a vertically integrated laser diode and waveguide photodetector. All three major components of the sensor were integrated in a single 7.1 µm thick molecular beam epitaxy (MBE) epitaxial growth, lattice matched to an InP substrate. Full fabrication of the integrated MEMS device utilizes 7 projection lithography masks, 4 nested inductively coupled plasma (ICP) etches, and over 60 discrete processing steps. This dissertation focuses on the integration design and the development of specific III-V semiconductor fabrication processes in order to completely fabricate and realize these devices, including specialized ICP etching steps and a MEMS undercutting release etch. The fabricated devices were tested and characterized by investigating the separate component subsystems as well as the total combined system performance. Investigation of device failure and performance degradation is performed and related to non-idealities in the device fabrication and design. A discussion of future work to improve the performance of the system is presented. The work in this dissertation describing the successful fabrication process and analysis of such a complex system is a milestone for III-V based optical MEMS research and will serve as the groundwork for future research in the area of optical MEMS microsystems.