This study examined whether decoding and linguistic comprehension abilities, broadly defined by the Simple View of Reading, in grade 1 each uniquely predicted the grade 6 writing performance of English‐speaking children (n = 76) who were educated bilingually in both English their first language and French, a second language. Prediction was made from (1) English to English; (2) French to French; and (3) English to French. Results showed that both decoding and linguistic comprehension scores predicted writing accuracy but rarely predicted persuasive writing. Within the linguistic comprehension cluster of tests, Formulating Sentences was a strong consistent within‐ and between‐language predictor of writing accuracy. In practical terms, the present results indicate that early screening for later writing ability using measures of sentence formulation early in students’ schooling, in their L1 or L2, can provide greatest predictive power and allow teachers to differentiate instruction in the primary grades. Theoretically, the present results argue that there are correlations between reading‐related abilities and writing abilities not only within the same language but also across languages, adding to the growing body of evidence for facilitative cross‐linguistic relationships between bilinguals’ developing languages. This 5 year longitudinal study predicted writing in English and French in grade 6 from within‐ and cross‐language word decoding and oral linguistic comprehension abilities in Grade 1. Results showed significant predictive power for both factors, and more particularly for tasks involving sentence formulation in grade 1, suggesting cross‐language facilitation.