The cloud top height of marine boundary layer clouds (MBLCs) in the mid-latitudes has received less attention than that of subtropical MBLCs and is investigated here using cloud mask data, which were based on observations from the cloud–aerosol lidar and infrared pathfinder satellite observation (CALIPSO) satellite. This study provides a comprehensive overview of the observational characteristics of variations in cloud top height of MBLCs and fog frequency over the mid-latitudes. Seasonal variations in the cloud top height of mid-latitude MBLCs as well as the differences in these seasonal variations between the Northern and Southern hemispheres were determined. For example, over the North Pacific, the cloud top height is high in winter (up to 1800 m) but low in summer (down to 800 m), whereas in the Southern Hemisphere, the seasonal variation is not well defined, with heights ranging from 1300 to 1500 m. While clear seasonal variations in the frequency of fog occurrence are found over the North Pacific and the northwest Atlantic, the fog frequency over the Southern Ocean is almost constant irrespective of the season. High correlations were found between the MBLC top height and stability indexes and between the fog frequency and some surface parameters such as temperature difference between the surface air and the sea surface. The latitudinal variations in the cloud top height of MBLCs in summer and winter over the Southern Ocean were compared with those over the North Pacific. The difference in cloud top heights between nighttime and daytime is also presented.