Offspring traits are greatly influenced by maternal effects and these maternal effects may provide an important pathway through which populations can adapt to changing thermal environments. We investigated the effect of egg size on the among and within population variation in early life history traits among introduced Great Lakes Chinook salmon (Oncorhynchus tshawytscha) populations under varying thermal conditions. We reared Chinook salmon from three populations in a common garden hatchery study at 6.5°C, 9.4°C, and 15.2°C, and measured a variety of fitness-related traits during development. We found that most of the among population variation in early life history traits was explained by egg size. However, the contribution of egg size to the among population variation decreased with an increase in temperature suggesting that other effects, such as genetic, contribute at high temperature. Within populations, egg size explained much of the dam variance and maternal effect for traits in every temperature, whereas egg size generally had little to no influence on the sire variance and heritability. Overall, our results demonstrate the significant contribution egg size makes to shaping early life history phenotypes among and within populations, and suggests that egg size is an important pathway through which offspring phenotypes can evolve on contemporary timescales.