(1) Background: This study evaluated the efficacy of magnesium nanoparticles (MgNPs) synthesized through a green method utilizing bacterial metabolites (BMs) produced by Escherichia coli. (2) Methods: BMs were tested for total phenolic content by high-performance liquid chromatography. MgNPs were characterized by X-ray diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy, photoluminescence, and ultraviolet–visible spectroscopy. MgNPs and BMs were tested for antibacterial and antibiofilm potentials against multidrug-resistant clinical isolates by agar well diffusion, minimum inhibitory and bactericidal concentration assays, time–kill test, and inhibition of biofilm formation and destruction of pre-formed biofilm assays. Furthermore, they were tested for antioxidant potential by 2,2-diphenyl-1-picryhydrazyl radical scavenging assay. (3) Results: BMs included carbohydrates, reducing sugars, and phenols (gallic acid and catechin) with a total phenolic content of 0.024 mg GAE/g. MgNPs showed a pure crystalline structure with a spherical shape, 17.8 nm in size, and a 4.19 eV energy gap. Bacteria included Streptococcus pneumonia, Enterococcus faecium, Klebsiella pneumonia, and Salmonella Typhimurium. The antibacterial results showed inhibition zones ranging between 7.2 and 10.4 mm, a bactericidal effect of MgNPs, a bacteriostatic effect of BMs, and growth inhibition after 3 h. The antibiofilm results demonstrated significant inhibition of biofilm formation (inhibition percentages of 64.931% for MgNPs and 71.407% for BMs). However, the assays revealed modest biofilm destruction (eradication percentages of 48.667% for MgNPs and 37.730% for BMs). Antioxidant capacity revealed notable scavenging activity of MgNPs (scavenging activity of 41.482%) and weak activity of BMs (scavenging activity of 16.460%). (4) Conclusions: These findings support the application of MgNPs in biomedical fields.