In this study, we investigated longitudinal reciprocal relations among reading, executive function, and social-emotional skills in students from Grades 2 to 5, using the data set from the Early Childhood Longitudinal Study, Kindergarten Class of 2010-2011. We addressed several important gaps in the literature on longitudinal reciprocal relations by using latent factors to represent the executive function and social-emotional skills in latent growth models with structured residuals, separating between- and within-person effects, and examining sample effects with a general population sample, students with reading difficulties, and high-performing students. Our results showed longitudinal reciprocal relations between reading and executive function in high-performing students, such that with development, the contribution of executive function to reading grew stronger while the contribution of reading to executive function remained stable; we found no longitudinal reciprocal relations between reading and social-emotional skills in any of the three sample groups; and these patterns of results remained the same based on the control of socioeconomic status and sensitivity analyses. Together, the results of this study support the heterogenous hypothesis of mutualism theory in education: The effect of mutualism may be stronger in some contexts and populations than in others. Longitudinal reciprocal relations between executive function and reading may be driven mostly by high-quality and intensive learning and practice in reading, not by socioeconomic status.