Garcinia quaesita and Garcinia zeylanica are Sri Lankan endemic plants with significant therapeutic potential and numerous health-care applications. Despite this, there are no adequate literatures reported on the chemical compositions (CCs) and antioxidative potential (AP) of leaves' essential oils (EOs). The purpose of this study was to extract EOs from the leaves and investigate the CCs and AP of the extracted EOs. The hydro-distillation technique was used to extract the EOs, and the CCs of the EOs were identified through gas chromatography–mass spectrometry analysis. Only those compounds that had a matching value of more than 90% were taken into consideration, and the AP of the extracted EOs was determined using the ferric-reducing antioxidant power (FRAP) assay. Hydro-distillation process yielded EOs in the same quantity, 0.12% (v/w) on a fresh weight basis for two varieties. About 33 CCs that were found in the extracted EOs were mainly sesquiterpenes. The most prevalent substances in the EOs were copaene (19.39%), caryophyllene (12.94%), alloaromadendrene (12.12%), α-humulene (11.24%), and α-cubebene (9.38%). It is interesting to note that copaene and alloaromadendrene were only found in G. quaesita, whereas α-cubebene was only found in G. zeylanica. Caryophyllene and α-humulene were identified in both EOs at different concentrations. The EO from G. quaesita showed high AP, presenting FRAP values 274.74 ± 1.32 μL Trolox Eq/L. This study is recognized as being the first to examine the CCs and AP of EOs, and the results may inspire the creation of new uses and high-value leaf products.