The DISC1 protein is implicated in major mental illnesses including schizophrenia, depression, bipolar disorder and autism. Aberrant mitochondrial dynamics are also associated with major mental illness. DISC1 plays a role in mitochondrial transport in neuronal axons, but effects in dendrites have yet to be studied. Further, the mechanisms of this regulation, and its role in neuronal development and brain function are poorly understood. Here we demonstrate that DISC1 couples to the mitochondrial transport and fusion machinery via interaction with the outer mitochondrial membrane (OMM) GTPase proteins, Miro1 and Miro2, the TRAK1 and TRAK2 mitochondrial trafficking adaptors, and the mitochondrial fusion proteins Mitofusins. Using live cell imaging, we show that disruption of the DISC1 Miro/TRAK complex inhibits mitochondrial transport in neurons. We also show that the fusion protein generated from the originally described DISC1 translocation (DISC1-Boymaw) localises to mitochondria where it similarly disrupts mitochondrial dynamics and decreases ER-mitochondria contact area. Moreover, disruption of mitochondrial dynamics by targeting the DISC1-Miro/TRAK complex or upon expression of the DISC1-Boymaw fusion protein impairs the correct development of neuronal dendrites. Thus, DISC1 acts as an important regulator of mitochondrial dynamics in both axons and dendrites to mediate transport, fusion and cross-talk of these organelles, and pathological DISC1 isoforms disrupt this critical function, leading to abnormal neuronal development.