Most evidence concerning the development of early expressive lexicons has demonstrated that both intrinsic and environmental factors play important roles on the individual variations in the developmental patterns. One of the environmental factors is the variation in the language systems. The purpose of this study is to investigate the developmental changes in the semantic content and composition of early expressive vocabulary in Mandarin-speaking infants and toddlers. Parent report data (N = 1897) was collected using the Mandarin-Chinese Communicative Development Inventory (MCDI), and the results showed that the trend in the development of early expressive lexicons in Mandarin-speaking infants and toddlers were similar to those with other language backgrounds. The semantic content of the early expressive vocabulary was mostly related to the toddlers’ daily life, and the major grammatical category was nouns. Different developmental trends were found for different categories: nouns, verbs, and adjectives grew rapidly in the early stage while closed-class words showed a slow and continuous growth across the development of expressive vocabulary. In addition to the theoretical aspect, the detailed list of the age of acquisition for individual items provided in this study could be used to examine vocabulary development in children with language disorders and to design intervention programs in clinical settings.