This study investigated the effects of elevated temperature on the phenology and morphology of the early-maturing peach cultivar ‘Mihong’. The experiment was conducted from 2019 to 2024 in a temperature-gradient chamber at the National Institute of Horticultural and Herbal Science, Wanju, Korea, with four warming treatments (+2.2 °C to +5.0 °C above ambient). Higher temperatures delayed the onset of endodormancy and markedly shortened the period from endodormancy release to full bloom. Elevated temperatures also increased the LD ratio, with the proportion of fruits exceeding an LD ratio of 1.0 rising significantly with temperature. The LD ratio showed strong correlations with November mean temperature (MT11) and March maximum temperature (HT3) (r = 0.81) and was also associated with the average temperature (Temp3, r = 0.51) and duration (P3, r = −0.54) of the endodormancy release to full bloom phase. Stepwise and PLS regression identified temperatures in May, November, and March as key predictors of the LD ratio, while PCA revealed that temperature variables (Temp3, Temp5) and stage durations (P3, P4) were major contributors. These results confirm that climate warming alters the phenology and morphology of ‘Mihong’, reducing fruit quality and marketability, while providing a basis for predictive modeling and highlighting the importance of adaptive strategies such as shading or growth regulator application.