Transport characteristics of gas bubbles, suspended in magnetically activated ferrofluids, such as their shape and size, formation frequency and superficial velocities, can be effectively manipulated by strategically imposing an external magnetic field in the flow domain. We demonstrate and analyse such a flow manipulation technique on an air-ferrofluid Taylor slug-bubble flow, formed in a T-junction mini-channel fluidic device having a square cross-section of sides 2 mm. The flow transport characteristics get affected by an interplay between strength of the externally applied magnetic field, local magnetic pressure barrier due to the induced magnetic force, the concentration of suspended magnetic particles and the flow Reynolds number. The applied magnetic field, depending on its location and strength with respect to the T-junction, tends to cause a hump-shaped agglomerate or a flocculate of the suspended nano-particles in the flow. This alters the dynamics of the resulting Taylor bubbles getting generated at the T-junction, providing the required means of flow manipulation and control. Taylor bubble formation at the T-junction and the effect of magnetic field on its dynamics is analysed. A parametric investigation is also performed to study the effect of air-ferrofluid flow rate ratio and magnetic force on the resulting hydrodynamics of the Taylor bubble flow. Such magnetic manipulation of air-ferrofluid flow can be effectively utilized in technological applications such as chemical reactors for mass transfer augmentation, pulsating heat pipes and lab-on-chip devices, to name a few.