Mushrooms can be employed as functional foods or medicines for the prevention and treatment of several chronic diseases, since they are rich sources of bioactive metabolites. Nonetheless, the therapeutic value of Algerian mushrooms remains largely unexplored. To date, there are no published studies concerning the different medicinal properties of endemic Ganoderma lucidum in Algeria. This study aimed to compare the antioxidant potential of an endemic specimen collected in the Lake Tonga Forest of El Kala National Park in Northeast Algeria with findings from other countries. To this end, phenolic and flavonoid contents were determined using three solvents of increasing polarity (chloroform, ethyl acetate, and butanol). The mushroom specimen was identified as G. lucidum based on strong molecular evidence using internal transcribed spacer, large subunit, and small subunit sequences. Our study showed that ethyl acetate extract contained higher total phenolic content (171.1 ± 1.06 μg gallic acid equivalents/mg and 102.5 ± 0.69 μg pyrocatechol equivalents/mg extract) and total flavonoid content (25.48 ± 0.13 μg quercetin equivalents/mg and 40.45 ± 0.83 μg rutin equivalents/mg extract) than the other mushroom extracts (chloroform and butanolic). The ethyl acetate extract also showed good antioxidant activity in DPPH (28.51 ± 0.24 μg/mL), ABTS·+ (10.06 ± 0.13 μg/mL), galvinoxyl radical (15.46 ± 0.48 μg/mL), reducing power (22.74 ± 0.30 μg/mL), cupric reducing antioxidant capacity (21.36 ± 0.04 μg/mL), and phenanthroline (12.87 ± 0.26 μg/mL) assays. These results suggest that G. lucidum could be used as a source of strong natural antioxidants for the food and pharmaceutical industries.