The continuous emission of CO2 from anthropogenic activities poses a severe challenge to environmental sustainability. Activated carbon from bio-wastes has evolved as a cost-effect and efficient adsorbent for sequestering harmful gases like CO2. This study investigates the adsorption of CO2 using composite activated carbon from almond and mango seed shells. Three (3) different samples, including the activated almond seed shell (AC-KA), the mango seed shell (AC-KM), and the almond-mango seed shell composite (AC-KAM), were developed using KOH as an activating agent at 380 °C and characterized to determine their suitability as adsorbents. The nitrogen adsorption–desorption isotherms indicate that the activated carbon samples are type I microporous solids. The BET results showed that the AC-KM had the highest specific surface area of 629 m2/g and an average pore diameter of 2.12 nm, followed by the composite AC-KAM sample with a BET surface area and pore diameter of 380 m2/g and 1.78 nm, respectively. The SEM–EDX results showed that the composite AC-KAM had the highest atomic carbon percentage of 59.23%, making it a relatively good source of activated carbon. The FTIR analysis indicated the presence of similar functional groups, especially the primary amine group, with a high affinity for acid gases, in all three samples. The CO2 adsorption experiment results showed that the Langmuir equation adequately modeled the AC-KAM adsorption process, with an adsorption capacity of 1.6245 mmol/g at 26 °C. A Freundlich exponent of n > 1 showed that the adsorption process was favorable. Therefore, the AC-KAM sample can be used as an adsorbent for ambient low-concentration CO2 capture. [ABSTRACT FROM AUTHOR]