Nepeta sibirica L. or Siberian catmint is a medicinal plant species used in Mongolian traditional medicine for curing human different disorders and veterinary practices. The previous study of the whole plant concentrated on the determination of its essential oil composition and reported that the major ones are sesquiterpenes, including nepetalactone. The aim of this study was to reveal a new biological activity of the above-ground parts of N. sibirica L. and compare the activity of different extracts correlating with the content of biologically active compounds and evaluate their toxicity. For this purpose, anti-oxidative and acetylcholinesterase inhibitory activities of the above-ground parts of N. sibirica L. aqueous and ethanol (EtOH) (40%, 70%) extracts were assayed spectrophotometrically. The aqueous extract showed positive anti-oxidative activity by both tested DPPH and FRAP assays with IC 50 134.24 ± 1.42 mg/mL and FRAP value 1385.15 ± 8.12 μmol/L at 200 μg/mL, in contrast to 40% and 70% EtOH extracts. The 70% EtOH extract presented the highest acetylcholinesterase inhibitory activity (IC 50 77.29 ± 0.38 mg/mL) followed by 40% EtOH extract (176.72 ± 0.35 mg/mL) and aqueous extract (275.41 ± 0.23 mg/mL). Total phenolics were found to be gallic acid equivalent, % 3.74 ± 0.05 (70% EtOH), 3.94 ± 0.04 (40% EtOH), and 3.79 ± 0.16 (aqueous), whereas the total flavonoids as a rutin equivalent, % as 2.01 ± 0.12, 1.44 ± 0.17 and 1.99 ± 0.02, each. The aqueous extract showed the best antioxidative and lowest activity against the acetylcholinesterase; however, the 70% EtOH extract showed the opposite effects than that of the aqueous. No mortality incidence was visible at various doses, indicating that the oral median lethal dose of aqueous and 70% EtOH extracts were considered greater than 5000 mg/kg. N. sibirica L. belongs to the non-toxic category of the OECD 423 classification.