Magnesium Chloride is 

A compound is a chemically bound group of two or more atoms of different elements that are electrically neutral.
Since Magnesium Chloride $\left({\mathrm{MgCl}}_2\right)$ is made up of atoms from two different elements magnesium $\left(\mathrm{Mg}\right)$ and chlorine $\left(\mathrm{Cl}\right)$, it is a compound.