Due to its abundant content of biologically active compounds, Althaea officinalis L. (marshmallow) has been extensively researched and applied in both the food and pharmaceutical industries. This study aimed to evaluate hydroethanolic and natural deep eutectic solvent (NADES) extracts from leaves, flowers, and roots in terms of their chemical composition and biological activities. Extracts were characterized using chromatographic and spectrophotometric methods. Phytochemical profiling by HPLC-MS revealed a diverse composition, with 35 secondary metabolites identified or tentatively assigned, mainly hydroxycinnamic acid derivatives and flavonoid glycosides. GC-MS analysis of the ethanol extracts identified ten free amino acids, seven organic acids, several mono- and disaccharides, and one oligosaccharide. Their concentrations varied across different parts of the plant depending on the specific metabolism of the respective organ. Ethanolic extracts showed the highest total phenolic content (up to 176 mg GAE/L), while flower extracts exhibited the strongest antioxidant activity (DPPH up to 89 µmol TE/L). The antimicrobial potential of the extracts was assessed by the agar well diffusion method. NADES1 extracts demonstrated significant antibacterial activity, with inhibition zones reaching up to ~34 mm, whereas NADES2 extracts were largely inactive. In contrast, antifungal activity was negligible or absent across all samples.