Microtubules are polymers of tubulin heterodimers that exhibit dynamic instability: periods of growth followed by periods of shrinkage. However, the molecular regulation of dynamic instability remains elusive. Here, we show that EFA-6, a cortically-localized protein, limits the growth of microtubules near the cell cortex of early embryonic cells from Caenorhabditis elegans, possibly by inducing microtubule catastrophes. Compared with wild type, embryos lacking EFA-6 had abnormally long and dense microtubules at the cell cortex, and growing microtubule plus ends resided at the cortex for up to five-fold longer. Loss of EFA-6 also caused excess centrosome separation and displacement towards the cell cortex early in mitosis, and subsequently a loss of anaphase spindle-pole oscillations and increased rates of spindle elongation. The centrosome separation phenotype was dependent on the motor protein dynein, suggesting a possible link between the modulation of microtubule dynamics at the cortex and dynein-dependent force production. EFA-6 orthologues activate ARF6-type GTPases to regulate vesicle trafficking. However, we show that only the C. elegans EFA-6 amino-terminus is both necessary and sufficient to limit microtubule growth along the cortex, and that this function is independent of ARF-6.