In this study, the changes of long-term mean annual water levels of the Great Lakes in North America are investigated, including the potential impacts of the North Atlantic Oscillation (NAO). The levels of Lakes Superior, Michigan-Huron, St. Clair, Erie and Ontario and the hydro-climatic data, such as precipitation, evaporation and runoff, are evaluated together in view of the influence of NAO. The changes in the lake levels and hydro-climatic indicators are analysed. The annual hydro-climatic data, annual mean lake levels and the NAO indices are nondimensionalized through the nonparametric Mann-Kendall test. Then, the sequential Mann-Kendall test and paired t test are performed to gain insight into the trends of the time series and possible turning points. As a result of this study, the trend direction changes for all of the lakes are determined in the years 1965 and 1987, based on the records of the last 95 years. A similar tendency for evaporation, precipitation and runoff is found in 1982, 1935 and 1965. Regarding the effect of North Atlantic Oscillation on the changes of the lake levels, the same directional variations are found between the lake levels and the NAO FMA (February-March-April) indices. However, the directional variations between the lake levels and NAO JJA (June-July-August) indices are reversed. The absolute values of the correlation coefficients increase from west to east (from Lake Superior to Lake Ontario).