Red mud (RM) and electrolytic manganese residue (EMR) possess inherently complementary acid–base characteristics, and their synergistic utilization offers a promising approach to simultaneously enhance mechanical performance and mitigate environmental risks. However, the environmental behavior of RM-EMR composites, particularly in terms of pH buffering, ammonium nitrogen (NH₄⁺-N) control, and heavy metal mobility, remains inadequately understood. In this study, a composite cementitious paste was developed using RM and EMR, and the effects of the RM-to-EMR ratio (1: 3, 2: 3, 1: 1, and 3: 2), alkali activator dosage (30%, 40%, 50% and 60% by weight), and curing time (3 day, 7 day, 14 day, and 28 day) under standard curing conditions on unconfined compressive strength (UCS) were systematically evaluated. Leaching tests were conducted to assess pH evolution, NH₄⁺-N release, and heavy metal migration. The results showed that the optimal 28-day UCS of 35 MPa was achieved with an RM-to-EMR mass ratio of 2:3 and an activator dosage of 60%. EMR contributed to NH₄⁺-N leaching concentrations as high as 302 mg/L; however, under alkaline conditions (pH > 11), over 50% of the block samples met regulatory limits due to the transformation of NH₄⁺ into gaseous NH₃. Furthermore, Mn and Cd were effectively immobilized. In contrast, Al and Se exhibited elevated leaching, with Al showing particularly high concentrations under water leaching conditions. These results underscore the importance of raw material pretreatment and system optimization. Overall, this study provides new insights into the environmental behavior and safe resource utilization of RM and EMR in cementitious systems.