The effect of electrospun cellulose nanocrystals (CNCs)-polyetherimide (PEI) hybrid nanofibrous mats on Mode I and Mode II interlaminar fracture toughness of unidirectional carbon/epoxy composite laminates is demonstrated. It is shown that the CNCs reinforced PEI nanofibrillar interleaves result in a ~28% increase in Mode I initial fracture toughness values compared to neat PEI nanofibrous interleaves. Specifically, the interrelated micro- and nano-scale toughening mechanisms including carbon fibre bridging, fibre necking, fibre rupture with CNCs aggregates, and nanofibre rupture contributed to the fracture toughness improvements under Mode-I loading. Nano-scale mechanisms of shear hackles, and crack pinning by CNCs aggregates increased the Mode II fracture toughness up to ~3 kJ/m2 as a result of a 6 wt.% CNCs reinforced PEI nanofibrillar mat interleaves. Interleaving laminated composites with electrospun CNCs-PEI hybrid nanofibrillar mats has been demonstrated as a novel and prospective strategy to strengthen and toughen interlaminar zones of carbon/epoxy composite laminates.