Ultrasonic pretreatment of HTC precursor solution (sucrose) has been used to control the size of synthesized carbon nanospheres (CNSs). The pretreatment allowed for synthesis of monodispersed CNSs without yield compromise or additional chemical complexity. Based on ultrasonic cavitation intensity and energy, a mechanism for observed size change with changing ultrasonic pretreatment duration has been proposed. To compare the effect of ultrasonic pretreatment on physicochemical properties of samples, treated and untreated samples were analyzed using SEM, XRD, UV–Vis, FTIR, BET, and pH_pzc. The effect of ultrasonic pretreatment on the applicability of CNSs for adsorption and photodegradation of model pollutant systems was studied using cationic dyes methylene blue (MB) and crystal violet (CV). The fitted kinetic models exhibited that the adsorption was taking place by chemisorption for treated and untreated samples. The photodegradation studies were done under UV and visible light conditions with % decolorization followed: UV light > visible light > dark. The degradation mechanism has been explained using scavenger tests. The results reveal that ultrasonic pretreatment has no influence on sample’s physicochemical characteristics, adsorption, or degradation mechanisms. Hence, this opens up a new, convenient, and inexpensive technique for reducing size of HTC CNSs.