Using data from a longitudinal German large-scale study and structural equation modeling, this study investigated the effects of different dimensions of the early home learning environment (HLE) as well as the effect of language-related preschool quality at the age of 3 to 5 years on students' later school-relevant language proficiency (as indicated by their school-relevant listening comprehension) in primary school (N = 316; M[subscript age in months at Grade 1] = 88.22, SD = 3.98). Results show significant direct effects of both the book exposure at home as well as of the language and literacy-related preschool quality on later school-relevant language proficiency, even when controlling for earlier language skills at preschool entry (age 3/4) and other relevant child (working memory, nonverbal cognitive abilities) and family-related (socioeconomic status) factors. Using child vocabulary and grammar at age 5/6 as mediational variables, the effect of the informal HLE was partially mediated through students' receptive grammar skills. Our results underline the importance of early stimulating home and preschool literacy environments for students' school-relevant language proficiency at the start of primary school.