Although studies have consistently found negative correlations between second language (L2) anxiety and L2 achievement, the anxiety concept is criticized for the failure to consider confounding variables, particularly first language (L1) achievement, in the relationship between language anxiety and language achievement. Also, past studies tend to rely on cross-sectional data to conclude causality. To assess long-term causal relationships between L1 and L2 achievement, the study used structural equation modeling to examine pathways by which L1 achievement is associated with L2 reading anxiety after two years via several mediators: L1 working memory, L1 print exposure, and L1 metalinguistic knowledge; L2 aptitude (MLAT); and L2 reading, writing, and listening comprehension. Students (n = 293) in a U.S. secondary school were administered these measures and followed through first-year Spanish. Findings showed the effect of L1 achievement on later L2 reading anxiety was direct and indirect through L2 aptitude, L2 achievement, and L1 metalinguistic knowledge. Effects of L2 achievement and L1 metalinguistic knowledge on L2 reading anxiety were direct. An out-of-sample analysis for model prediction power supported the external validity of the study results. We recommend that teachers and learners deal with L1/L2 skills, rather than anxiety per se, in order to reduce sense of L2 reading anxiety.