For the past 26 years, the speaker has been engaged in the design, implementation, technology transfer, and application of flexible Mandatory Access Control (MAC). In this talk, he describes the history and lessons learned from this body of work. The background and motivation for MAC is first presented, followed by a discussion of how a flexible MAC architecture was created and matured through a series of research systems. The work to bring this architecture to mainstream systems is then described, along with how the architecture and implementation evolved. The experience with applying this architecture to mobile platforms is examined. The role of MAC in a larger system architecture is reviewed in the context of a secure virtualization system. The state of MAC in mainstream systems is compared before and after our work. Work to bring MAC to emerging operating systems is discussed.