Many Latino children in the U.S. speak primarily Spanish at home with few opportunities for exposure to English before entering school. For monolingual children, the strongest early predictor of later school success is oral language skill developed before kindergarten. Less is known about how early oral language skills support later learning in sequential bilingual children. A question with wide-reaching significance is whether skill in a child's first language (L1) supports later learning in a second language (L2). In this longitudinal study of sequential Spanish-English bilinguals, we assessed oral language skills in Spanish at 2 years through parent reports of vocabulary size and children's real-time language processing efficiency (Accuracy, RT) in the 'looking-while-listening' (LWL) task. At 4½ years, we assessed language outcomes in both Spanish and English using standardized tests. Reported relative exposure to each language was significantly correlated with language outcomes in Spanish and English. Within-language relations were observed between Spanish vocabulary size and processing efficiency at 2 years and later Spanish-language outcomes. Critically, across-language relations were also observed: Children with stronger Spanish-language processing efficiency at 2 years had stronger English-language skills at 4½ years, controlling for socioeconomic status and exposure to English. Children's early language processing efficiency in Spanish is associated with stronger real-time information processing skills that support maintenance of Spanish and learning in English when these children enter school. These results support the recommendation that primarily Spanish-speaking families should engage in activities that promote children's Spanish-language skills while also seeking opportunities for children to be exposed to English.