We examine the unintended long-term consequences of the U.S.'s role as a dollar lender of last resort. Using administrative data, we show that dollar swap lines substitute for both market-based dollar funding and foreign central banks’ dollar reserves, especially when those sources are scarce. However, this backstop incentivizes global banks to take on greater currency mismatches while prompting foreign central banks to hold fewer dollar reserves, such as U.S. Treasuries. We develop a model that formalizes intermediated dollar lending of last resort, highlighting the global intermediation chain in emergency dollar provision. In the model, while swap lines help stabilize dollar appreciations and mitigate CIP deviations during times of stress, they lead to ex-ante over-investment (under-investment) in dollar assets by foreign private (central) banks due to pecuniary externalities. This inefficiency introduces unintended risks not only for foreign economies but also, perhaps surprisingly, for the U.S., by shifting Treasury holdings toward less stable investors and raising fire-sale risks.