This article documents how customary land tenure systems mediate transformations of land use and livelihoods in the arid pre-Saharan region of southeast Morocco. Although dominant development paradigms for land governance have begun to recognize customary regimes, development orthodoxy still holds that formalizing rules as part of national property law is essential to ensuring economic growth or effectively governing land use and livelihood transformations. Our research in southeastern Morocco indicates that customary tenure regimes play a central role in managing such transformations in the absence of effective formal property frameworks. Extended ethnographic fieldwork in the Mgoun Valley of Tinghir province established the trajectory of rural livelihoods, migration, and land use in the latter half of the 20th Century. Additionally, the authors conducted a survey with 306 households in the Mgoun Valley, documenting income, asset, and livelihood profiles of households today; the survey produced demographic, livelihood, and asset profiles of these same households in the early 1960 s. The results confirm a marked shift from pastoralism to other land uses and livelihoods at the household level in the last half-century. The household-level data also show how the “invisible rules” embedded in customary tenure regimes shape livelihood and land use options available to different households. Unformalized tenure rights in collectively-owned land and breakdowns in customary resource management regimes under global economic pressures are often seen—in Morocco and other parts of the world—as producing chaotic or unsustainable land use change. However, this research shows that customary land tenure may, in fact, actively manage such change using rules that may be invisible to formal data collection and policy-making institutions, but are not invisible to the people negotiating these dynamic rule systems.