Both early childhood maternal language input and the quality of classroom instruction in elementary school have been shown to be important environmental supports in predicting children's literacy skill development. However, no studies have simultaneously examined these two environmental supports in relation to children's early language skills and later literacy skills across elementary school. The current study examined how multiple years of early maternal language input from 6 to 36 months and later classroom instructional quality from pre-kindergarten (pre-K) through fifth grade were related to children's early language at 36 months and later literacy trajectories in word recognition and reading comprehension across elementary school. The study included a diverse, population-representative sample of 1,292 children who were born in low-wealth rural communities and followed through fifth grade. Video recordings and subsequent transcripts of mother-child shared picture book tasks in the home at four timepoints during early childhood were used to assess maternal diversity of vocabulary, utterance complexity, and engagement (wh-questions) during book sharing. The quality of instruction in elementary school was assessed using classroom observations from pre-K through fifth grade. Maternal complexity and engagement were indirectly related to children's literacy trajectories across pre-K to fifth grade through effects on early child language at 36 months. Higher-quality classroom instruction was not reliably related to concurrent literacy skills in pre-K through fifth grade. Findings suggested the enduring importance of early maternal language input in predicting children's early language and later literacy skill development during elementary school.