Astragalus species are traditionally used for diabetes, ulcers, leukemia, wounds, stomachaches, sore throats, abdominal pain, and toothaches. Although the preventive effects of Astragalus species against diseases are known, there is no record of the therapeutic effects of Astragalus alopecurus. In this study, we aimed to evaluate the in vitro antiglaucoma, antidiabetic, anti-Alzheimer’s disease, and antioxidant activities of the methanolic (MEAA) and water (WEAA) extracts of the aerial part of A. alopecurus. Additionally, its phenolic compound profiles were analyzed by liquid chromatography–tandem mass spectrometry (LC–MS/MS). MEAA and WEAA were evaluated for their inhibition ability on α-glycosidase, α-amylase, acetylcholinesterase (AChE), and human carbonic anhydrase II (hCA II) enzymes. The phenolic compounds of MEAA were analyzed by LC-MS/MS. Furthermore, total phenolic and flavonoid contents were determined. In this context, the antioxidant activity was evaluated by 1,1-diphenyl-2-picrylhydrazyl (DPPH), 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), N,N-dimethyl-p-phenylene diamine (DMPD), ferric reducing antioxidant power (FRAP), cupric ions (Cu²⁺) reducing antioxidant capacity (CUPRAC), ferric ions (Fe³⁺) reducing, and ferrous ions (Fe²⁺) chelating methods. MEAA and WEAA had IC₅₀ values of 9.07 and 2.24 μg/mL for α-glycosidase, 693.15 and 346.58 μg/mL for α-amylase, 1.99 and 2.45 μg/mL for AChE, and 147.7 and 171.7 μg/mL for hCA II. While the total phenolic amounts in MEAA and WEAA were 16.00 and 18.50 μg gallic acid equivalent (GAE)/mg extract, the total flavonoid contents in both extracts were calculated as 66.23 and 33.115 μg quercetin equivalent (QE)/mg, respectively. MEAA and WEAA showed, respectively, variable activities on DPPH radical scavenging (IC₅₀: 99.02 and 115.53 μg/mL), ABTS radical scavenging (IC₅₀: 32.21 and 30.22 µg/mL), DMPD radical scavenging (IC₅₀: 231.05 and 65.22 μg/mL), and Fe²⁺ chelating (IC₅₀: 46.21 and 33.01 μg/mL). MEAA and WEAA reducing abilities were, respectively, Fe³⁺ reducing (λ₇₀₀: 0.308 and 0.284), FRAP (λ₅₉₃: 0.284 and 0.284), and CUPRAC (λ₄₅₀: 0.163 and 0.137). A total of 35 phenolics were scanned, and 10 phenolic compounds were determined by LC-MS/MS analysis. LC-MS/MS revealed that MEAA mainly contained isorhamnetin, fumaric acid, and rosmarinic acid derivatives. This is the first report indicating that MEAA and WEAA have α-glycosidase, α-amylase, AChE, hCA II inhibition abilities, and antioxidant activities. These results demonstrate the potential of Astragalus species through antioxidant properties and enzyme inhibitor ability traditionally used in medicine. This work provides the foundation for further research into the establishment of novel therapeutics for diabetes, glaucoma, and Alzheimer’s disease.