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This study aimed to isolate and characterise endophytic bacteria from the pioneer plant <i>Ageratina adenophora</i> in a mining area. Seven strains of metal-resistant endophytic bacteria that belong to five genera were isolated from the roots of <i>A. adenophora</i>. These strains exhibited various plant growth-promoting (PGP) capabilities. <i>Sphingomonas</i> sp. ZYG-4, which exhibited the ability to secrete indoleacetic acid (IAA; 53.2 ± 8.3 mg·L⁻¹), solubilize insoluble inorganic phosphates (Phosphate solubilization; 11.2 ± 2.9 mg·L⁻¹), and regulate root ethylene levels (1-aminocyclopropane-1-carboxylic acid deaminase activity; 2.87 ± 0.19 µM α-KB·mg⁻¹·h⁻¹), had the highest PGP potential. Therefore, <i>Sphingomonas</i> sp. ZYG-4 was used in a pot experiment to study its effect on the biomass and Pb uptake of both host (<i>Ageratina adenophora</i>) and non-host (<i>Dysphania ambrosioides</i>) plants. Compared to the uninoculated control, <i>Sphingomonas</i> sp. ZYG-4 inoculation increased the biomass of shoots and roots by 59.4% and 144.4% for <i>A. adenophora</i> and by 56.2% and 57.1% for <i>D. ambrosioides</i>, respectively. In addition, <i>Sphingomonas</i> sp. ZYG-4 inoculation enhanced Pb accumulation in the shoot and root by 268.9% and 1187.3% for <i>A. adenophora</i>, and by 163.1% and 343.8% for <i>D. ambrosioides</i>, respectively, compared to plants without bacterial inoculation. Our research indicates that endophytic bacteria are promising candidates for enhancing plant growth and facilitating microbe-assisted phytoremediation in heavy metal-contaminated soil.