We report magnetic-field-driven on-demand manipulation of liquid metal in microfluidic channels filled with base or acid. The liquid metal was coated with iron (Fe) particles and treated with hydrochloric acid to have strong bonding strength with the Fe particles. The magnetic liquid metal slug inserted in the microchannel is manipulated, merged, and separated. In addition, corresponding to the repositioning of an external magnet, the liquid metal slug can be readily moved in microfluidic channels with different angles (>90°) and cross-linked channels in any direction. We demonstrated the functionality of the liquid metal in the microfluidic channel for electrical switching applications by manipulation of the liquid metal, resulting in the sequential turning on of light emitting diodes (LEDs).