Silica nanotubes (SNTs) functionalized with magnetic and up-conversion luminescent (UCL) NaYF₄:Yb/Er/Gd nanocrystals (NCs) (denoted as MUCNCs@SNTs) have been prepared by single-nozzle electrospinning based on a phase separation effect without any templates. Monodisperse and hydrophilic cubic α-NaYF₄:Yb/Er/Gd NCs decorated with polyethyleneimine (PEI) were fabricated in a facile hydrothermal route. Then, the Gd³⁺-doped α-NaYF₄:Yb/Er NCs were dispersed into the electrospinning precursor solution containing polyvinylpyrrolidone (PVP) and tetraethylorthosilicate (TEOS), followed by the preparation of precursor nanotubes via electrospinning process. Finally, after annealing at 600 °C, pure MUCNCs@SNTs were obtained. The biocompatibility test on L929 fibroblast cells using MTT assay reveals low cytotoxicity of the composites. Doxorubicin hydrochloride (DOX), a typical anti-cancer drug, was introduced into MUCNCs@SNTs to evaluate the loading and sustained release behaviours. The composite carriers provide pH-dependent drug release behaviour owing to abundant Si–OH active bonds of silica and its interactions with DOX. The in vitro cytotoxicity and cell uptake behaviour of the MUCNCs@SNTs for HeLa cells were evaluated. For in vitro magnetic resonance imaging (MRI), the composites show the promising spin lattice relaxation time (T₁) weighted effect and could potentially apply as a T₁-positive contrast agent. In addition, the composites show near-infrared UC luminescence and were successfully applied in the bioimaging of HeLa cells. Considering the good biocompatibility, high drug releasing content and pH-dependent drug release of the materials, these magnetic and luminescent composite nanotubes have potential applications in drug sustained release and magnetic resonance/UC luminescence modality imaging.