When mutant superoxide dismutase 1 (SOD1) was first linked to amyotrophic lateral sclerosis (ALS), it was thought that ALS is a disease of oxidative stress. Since the discovery of multiple ALS-linked genes involved in RNA processing and proteostasis, most recent research has been focused on molecular defects induced by those mutant genes, but whether and how oxidative stress might contribute to magnification of those molecular defects have remained unaddressed. Strikingly, a hallmark of both familial and sporadic ALS is the aggregation of the essential DNA/RNA binding protein TDP-43, which is thought to cause simultaneous loss-of-nuclear function and gain-of-cytoplasmic toxicity. A central unanswered question is what triggers ALS in adult followed by rapid and irreversible progression? Here we report that neuronal cells are particularly prone to oxidative stress to trigger TDP-43 aggregation, which in turn sponges a large subset of microRNAs and proteins to cause both up-regulation and functional depletion of different sets of gene products. Remarkably, a large fraction of those functionally perturbed genes are nuclear genome-encoded mitochondrial proteins, thus resulting in a global mitochondrial imbalance, which further augments oxidative stress. We propose that this ferocious cycle driven by oxidative stress in combination with causal mutations in critical genes may underlie ALS onset and progression.