Prominent theories of reading development have separately emphasized the relevance of children's skill in learning (Share, 2008) and lexical representations (Perfetti & Hart, 2002). Integrating these ideas, we examined whether skill in learning lexical representations is a mechanism that might explain children's reading development. To do so we conducted a longitudinal study, following 139 children from Grades 3 to 5. In Grade 3, children completed measures of word reading and reading comprehension and again at Grade 5. In Grade 4, children read short stories containing novel words; they were later tested on their memory for the spellings and meanings of these new words, capturing orthographic and semantic learning, respectively. Using multiple-mediation path analysis, we tested whether children's skill in learning orthographic and semantic dimensions of new words was a mediator of individual differences in each of word reading and reading comprehension. In models controlling for nonverbal ability, working memory, vocabulary, and phonological awareness, we found two clear effects: individual differences in orthographic learning at Grade 4 mediated the gains that children made in word reading between Grades 3 and 5 and individual differences in semantic learning at Grade 4 mediated gains in reading comprehension over the same time period. These findings suggest that children's ability to learn lexical representations is a mechanism in reading development, with orthographic effects on word reading and semantic effects on reading comprehension. These findings show the power and the specificity of children's capacity to learn in determining their progress in learning to read.