Ovarian cancer (OC) is the second most common genital cancer worldwide, and the most lethal of all genital cancers. The role of inflammation and markers of systemic inflammation such as neutrophils, lymphocytes and monocytes in cancer biology have been investigated and reported in many studies. Cancer antigen 125 (CA-125) is currently in use as an adjunct to diagnosis, prognostication and monitoring of epithelial OC (EOC). This test is not readily available in many centres in sub-Saharan Africa, creating a need to identify alternative markers that are available and affordable. This study aimed to determine the relationship between pre-operative serum lymphocyte to monocyte ratio (LMR) and CA-125 in EOC. This was a retrospective cross-sectional study among 70 women, diagnosed with EOC in Lagos University Teaching Hospital from January 2013 to December 2019. Data were extracted from the case notes of the patients. LMR was calculated as the absolute lymphocyte count divided by the absolute monocyte count and analysed using Statistical Package for Social Sciences (SPSS) version 25.0. The correlation between LMR and CA-125 was determined using Pearson’s correlation coefficient. The mean age of the patients was 48.57 ± 13.97 years. Serous adenocarcinoma was the most common subtype of EOC making up 94.3% of the cases. The median serum CA-125 was 393.5 (215.00–765.67) U/mL. The median LMR was 6.77 (1.28–43.0). There was a statistically significant negative correlation between CA-125 and LMR, r = −0.28, p = 0.02. LMR was negatively associated with CA-125 in women with EOC. LMR may be considered as a simple, affordable alternative marker to CA-125 in the management of EOC.