To assess the eastern Atlantic tropical mixed layer depth (MLD) at 4°W in the Gulf of Guinea, water temperature and density profiles from over five hundred historical observational Conductivity – Temperature - Depth (CTD) data were used. These data, obtained from key oceanographic survey databases covering 60 years (1956 to 2016) were used for a numerical and visual determination of the MLD. The numerical approach consists of the use of the algorithms of three methods; while the visual estimation of the MLD were made on both temperature and density profiles. The numerical approaches were evaluated by comparing their results on the determination of tropical MLD with those of the visual inspections by means of statistical and graphical analysis in order to determine the most suitable method for the determination of MLD in the study area. Our results show that the Boyer Montegut density threshold method (potential density) with the constant criterion Δσ = 0.03 kg.m−3 and a reference depth of ten meters is the most appropriate for determining the MLD in the Gulf of Guinea regardless the season. However, in the situation where only temperature profiles are available it is advisable to estimate MLD using the Lorbacher curvative method.