The aim of this study was to examine variation in early reading comprehension development for second language (L2) readers compared with first language (L1) readers and to investigate the impact of vocabulary knowledge in their first and second language. Participants were 75 Dutch monolingual children (L1 readers) and 71 Turkish-Dutch bilingual children (L2 readers), aged between 6 and 8 years old at the start of the study. In a longitudinal design, three waves of data were collected across second and third grades. The L2 readers had lower reading comprehension scores than the L1 readers on average, but this performance gap narrowed over time. To further investigate variation among the L2 readers, four categorical subgroups of L2 readers were identified with varying levels of L1 (Turkish) and L2 (Dutch) vocabulary knowledge by means of cluster analysis. Group membership was related to reading comprehension and showed an interaction with time, indicating that reading comprehension performance of the two L2 subgroups with high L1 vocabulary increased more over time compared with L1 readers. The L2 subgroup with high vocabulary in both languages even caught up with their monolingual peers in third grade. These findings demonstrate how individual differences in L1 and L2 vocabulary knowledge explain variation in early L2 reading comprehension development and highlight the importance of considering L2 readers' first language in research and education.