This study investigated the effectiveness of the Model of Reading Engagement (MORE), a content literacy intervention, on first graders' science domain knowledge, reading engagement, and reading comprehension. The MORE intervention emphasizes the role of domain knowledge and reading engagement in supporting reading comprehension. MORE lessons included a 10-day thematic unit that provided a framework for students to connect new learning to a meaningful schema (i.e., Arctic animal survival) and to pursue mastery goals for acquiring domain knowledge. A total of 38 first-grade classrooms (N = 674 students) within 10 elementary schools were randomly assigned to (a) MORE at school (MS), (b) MORE at home, (MS-H), in which the MS condition included at-home reading, or (c) typical instruction. Since there were minimal differences in procedures between the MS and MS-H conditions, the main analyses combined the two treatment groups. Findings from hierarchical linear models revealed that the MORE intervention had a positive and significant effect on science domain knowledge, as measured by vocabulary knowledge depth (effect size [ES] = 0.30), listening comprehension (ES = 0.40), and argumentative writing (ES = 0.24). The MORE intervention effects on reading engagement as measured by situational interest, reading motivation, and task orientations were not statistically significant. However, the intervention had a significant, positive effect on a distal measure of reading comprehension (ES = 0.11), and there was no evidence of Treatment × Aptitude interaction effects. Content literacy can facilitate first graders' acquisition of science domain knowledge and reading comprehension without contributing to Matthew effects.