Polyherbal preparations have gained much attention as a potential source for discovering new drug therapeutics for the treatment of diabetes mellitus. The present investigation aims to determine in vitro and in vivo antidiabetic activity and the antioxidant potential of hexane, ethyl acetate and methanol extracts of a polyherbal mixture prepared from equal amounts of garlic cloves (Allium sativum L.), curry leaves (Murraya koenigii L. Sprengel), black pepper seeds (Piper nigrum L.) and rath goraka fruits (Garcinia quaesita Pierre). The standardization and in vitro antioxidant activity and antidiabetic activities were determined using standard methods. The in vivo acute antihyperglycemic activity of the hexane, ethyl acetate and methanol extracts was determined using an oral glucose tolerance test in streptozotocin-induced diabetic Wistar rats. Highest significant in vitro antioxidant capacity in terms of DPPH free radical scavenging (24.71 ± 0.01 ppm, p = 0.02) and reducing antioxidant power of ferric ion (23.29 ± 0.78 mol/dm3, p = 0.01) and in vitro antidiabetic properties in terms of α-amylase (25.74 ± 0.60 ppm, p = 0.01) and α-glucosidase (22.37 ± 0.06 ppm, p = 0.02) inhibition activities were observed in the hexane extract when compared with the respective standard compounds, ascorbic for antioxidant (DPPH 6.78 ± 0.03 ppm; FRAP 25.02 ± 0.21 mol/dm3) and acarbose for antidiabetic activity (α-amylase 5.68 ± 0.35 ppm; α-glucosidase 17.11 ± 0.62 ppm). In the glucose tolerance test, a significant improvement of glucose tolerance was found in the hexane (17.38%) and ethyl acetate (15.81%) extracts-treated groups at the therapeutic dose against the diabetic control group (p < 0.05). The results obtained from the present evaluation showed that the hexane and ethyl acetate extracts of the polyherbal mixture could be considered as a potential source for developing antidiabetic agents targeting the management of diabetes mellitus.