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Understanding the pathogenic mechanisms of disease mutations is critical to advancing treatments. ALS-associated mutations in the gene encoding the microtubule motor KIF5A result in skipping of exon 27 (KIF5AΔExon27) and the encoding of a protein with a novel 39 amino acid residue C-terminal sequence. Here, we report that expression of ALS-linked mutant KIF5A results in dysregulated motor activity, cellular mislocalization, altered axonal transport, and decreased neuronal survival. Single-molecule analysis revealed that the altered C terminus of mutant KIF5A results in a constitutively active state. Furthermore, mutant KIF5A possesses altered protein and RNA interactions and its expression results in altered gene expression/splicing. Taken together, our data support the hypothesis that causative ALS mutations result in a toxic gain of function in the intracellular motor KIF5A that disrupts intracellular trafficking and neuronal homeostasis. [Display omitted] • ALS-associated KIF5A mutations result in a common novel toxic C-terminal sequence • ALS mutant KIF5A lacks autoinhibition resulting in a constitutively active kinesin • ALS mutant KIF5A displays a distal accumulation and altered axonal transport • ALS-associated KIF5A mutations result in novel protein and RNA interactions ALS-associated KIF5A mutations alter the C terminus, the effect of which had yet to be elucidated. Here, Baron et al. discover that these mutations impair KIF5A autoinhibition resulting in a hyperactive kinesin that displays altered protein function and aberrant cellular interactions. These observations shed light on the mechanisms contributing to ALS. [ABSTRACT FROM AUTHOR]