Purpose Oocyte in vitro maturation (IVM) is a patient-friendly reproductive technology but lower success rates than IVF have limited its uptake. Capacitation-IVM (CAPA-IVM) is an innovative new IVM system currently undergoing clinical evaluation. This study aimed to determine temporal effects of the pre-IVM phase of CAPA-IVM on cumulus function and oocyte developmental competence in mildly-stimulated mice. Methods Immature cumulus oocyte complexes (COCs) derived from mildly stimulated (23 h PMSG) 28-day-old mice underwent pre-IVM for 0–24 h in medium containing c-type natriuretic peptide (CNP), E 2 , FSH and insulin, prior to IVM (CAPA-IVM). The effect of pre-IVM duration on cumulus cell function and embryo development post-CAPA-IVM/IVF was assessed. Results Day 6 blastocyst rate increased incrementally with increasing pre-IVM duration: 40.6 ± 2.0%, 45.8 ± 1.2%, 52.2 ± 3.5%, 53.3 ± 5.9%, and 59.9 ± 2.5% for 0, 2, 6, 12, and 24 h pre-IVM, respectively ( P < 0.01). DNA content/COC, a measure of cumulus cell proliferation, was significantly higher with 24 h pre-IVM group compared to 0, 2, or 6 h pre-IVM ( P < 0.001). Pre-IVM for 24 h significantly increased cumulus expansion and mRNA expression of matrix genes Has2 and Tnfaip6 and Areg relative to no pre-IVM control ( P < 0.01). Cumulus-oocyte gap-junctional communication (GJC) was maintained throughout 24 h pre-IVM ( P < 0.0001), and GJC loss was slowed during the subsequent IVM phase, whilst meiotic resumption was accelerated ( P < 0.05). Pre-IVM increased COC ATP and ADP content ( P < 0.05), but not AMP, ATP/ADP, and energy charge. Conclusion The pre-IVM phase of CAPA-IVM improves the quality of IVM oocytes in a temporally dependent manner and significantly influences cumulus cell function including increased cell proliferation, cumulus expansion, and prolonged cumulus-oocyte GJC.