To mitigate the global climate crisis and address the growing demand for environmental monitoring, in this study, we developed Ni–Fe layered double hydroxides (LDHs) with molar ratios of 2:1, 2.5:1, and 3:1 as a multifunctional platform for CO2 capture, green hydrogen production, and advanced sensing applications. Characterization analysis confirmed the formation of ordered 2D nanosheets with an average size of ~300 nm, providing an optimal high-surface-area matrix for both catalytic and sensing interfaces. For carbon capture, the Ni–Fe LDH with a Ni:Fe molar ratio of 2:1 functionalized with 20 wt% tetraethylenepentamine achieved a peak uptake of 0.389 g CO2/g, significantly outperforming other compositions owing to Fe3+-enhanced Lewis acidity and hydroxyl density. Crucially, this extraordinary capture capacity translates into a massive mass-loading response, demonstrating its immense potential as an ultrasensitive recognition layer for gravimetric CO2 gas sensors. Regarding electrolysis, the Ni–Fe LDH-coated nickel foam with a Ni:Fe molar ratio of 2:1 yielded 50 mL of H2 over 15 h, representing an 11% improvement over bare nickel foam. Furthermore, this architecture maintained the lowest charge-transfer resistance (Rct), making it a highly promising conductive substrate for future electrochemical sensing. These results prove the 2:1 Ni:Fe ratio to be the optimal configuration for integrated carbon mitigation, clean energy production, and environmental monitoring technologies.