Current historiography on the making of the Tripoli-Tunisia border has focused on imperial diplomacy between France and the Ottoman Empire following the establishment of the French Protectorate over Tunisia in 1881. In such a way, the scholarship has emphasized the formal negotiations and disputes between Ottoman and French delegations to delineate a legal boundary between these two territories. Nevertheless, the current historiography has overlooked non-imperial actors' participation in making this border a fluid boundary—a boundary defined by the mobility of nomads, refugees, and locals who reshaped and undermined imperial conceptions of fixed borders. This paper examines the making of a mobile borderland from 1881 to 1893 when nomads and refugees resisted French and Ottoman attempts at delineating an official border between Tripoli and Tunisia. It consisted of two parts: 1) the mobility of nomads and refugees in destabilizing imperial conceptions of the Tripoli-Tunisia border and 2) the attempt to harness mobility to advance imperial interests. In particular, this paper inspects the activities of Tunisian and Tripolitanian nomadic tribes and refugees along the Tripoli-Tunisian border following the establishment of the French Protectorate over Tunisia. I argue that large mobile groups consisting of tens of thousands of nomads and refugees continually reshaped imperial conceptions of the Tripoli-Tunisian border and attested to new challenges and difficulties both empires had to address. Likewise, this paper contends that imperial tensions over the border evolved into attempts to harness nomadic tribes' and refugees' mobility as a destabilizing force to secure imperial interests. It demonstrates that mobility presented a dilemma: it was at once a detriment for empires that sought to settle recalcitrant, mobile populations and a boon if harnessed and directed towards undermining rival imperial interests. Overall, this paper offers a new perspective on making the Tripoli-Tunisia border and reassesses how non-elite actors played a constitutive role in shaping and redefining Ottoman and French imperial interests in late nineteenth-century North Africa.