A tree-ring width chronology of Mongolian pine (Pinus sylvestris var. mongolica) in northern China was established to analyze the tree growth response to climatic variation. The widths of Ringwood (RW), Earlywood (EW), and Latewood (LW) of Mongolian pine trees with the same wood density growing at Mu Us Desert and Saihanba National Forest Park were measured from tree-ring chronologies. Differences in the response of Mongolian pine trees to climatic variation were analyzed by measuring basal area increment. The results showed that the tree-ring width of Mongolian pine at Mu Us Desert presented a trend to initially increase and thereafter decrease with the increase in tree age, whereas at Saihanba tree-ring width showed a linear decreasing trend with tree age increase. The variation in basal area increment of Mongolian pine was similar to that of tree-ring width at the two study sites. The proportion of EW (as a percentage of RW) ranged from 65% to 70% at the two study areas, which indicated that the growth of EW made a greater contribution to RW than that conferred by LW growth. During the growth period, the relative percentages of EW and LW in the RW of Mongolian pine at Mu Us Desert showed only slight variation, whereas at Saihanba the percentage of EW decreased and the percentage of LW increased in the tree RW. The percentage of LW in the RW decreased during three drought events at Mu Us Desert. Correlation analysis showed that the radial growth of Mongolian pine at Mu Us Desert was positively correlated with the rainfall in April and July, the standardized precision evaporation index (SPEI) in July, the average temperature in December of the previous year and in March of the current year, and the minimum temperature in March and August, and was negatively correlated with the maximum temperature in November and June of the previous year. The radial growth of Mongolian pine at Saihanba was positively correlated with the rainfall in July and August, the SPEI, and the minimum temperature, and was negatively correlated with the maximum temperature in March and May of the current year. Structural equation modeling analysis showed that the annual average temperature and annual SPEI at Mu Us Desert had significantly negative effects on RW growth of Mongolian pine, the annual average rainfall had significantly positive effects on both EW and RW growth, and the annual minimum temperature and the annual average temperature had significantly positive and negative effects on LW growth, respectively. The responses of radial growth of Mongolian pine to the annually meteorological variables at Saihanba were generally similar to those observed at Mu Us Desert, but some differences were noted. The annual average rainfall had a strongly significant negative effect on LW of Saihanba, whereas the impact on EW did not attain significance. The drought stress effect of annual SPEI on RW and EW of Mongolian pine at Saihanba was significantly smaller than the impacts on RW and EW at Mu Us Desert.