Our laboratory recently identified the autophagy initiating kinase ULK1 as a novel AMPK substrate, which directly connects the highly conserved AMPK-mediated energy sensing pathway to the regulation of autophagy. Genetic deletion of ULK1 led to an accumulation of mitochondria with abnormal morphology, supporting a role for ULK1 kinase activity in the maintenance of mitochondrial homeostasis, yet few mammalian ULK1 substrates have been identified beyond those involved in the canonical initiation of autophagy. Here we have identified and validated the E3 ligase Parkin, a known mediator of mitochondrial quality control and the most commonly mutated gene in autosomal recessive early-onset Parkinson’s disease, as a direct ULK1 substrate. ULK1-mediated phosphorylation of Parkin was required for maximal Parkin activity in multiple assays of Parkin function. These data reveal an important role for ULK1-mediated Parkin phosphorylation in the regulation of mitochondrial homeostasis, as this modification seems to prime Parkin for maximal activity following depolarization. Further studies using mouse models have indicated that Parkin is phosphorylated in vivo in mouse livers and primary hepatocytes in response to activators of the AMPK/ULK1 pathway, including Metformin, the most widely prescribed drug for the treatment of Type II diabetes. These data suggest the potential for profound in vivo relevance for the regulation of Parkin function downstream of the AMPK/ULK1 signaling cascade.