Step 1: Write the electronic configuration of Hg.
Mercury (Z = 80) has the configuration \( [\text{Xe}] 4f^{14} 5d^{10} 6s^2 \), with the 5d and 4f subshells fully filled and the 6s subshell also completely filled with a paired electron pair.
Step 2: Check for unpaired electrons.
Since every subshell in this configuration is completely filled, there are no unpaired electrons anywhere in the atom, ruling out paramagnetism, ferromagnetism, and the presence of unpaired electrons.
Step 3: Conclude the magnetic behaviour.
A species with all its electrons paired is weakly repelled by an external magnetic field, which is the defining property of a diamagnetic substance.
\[ \boxed{\text{Hg is diamagnetic}} \]