Since $x=18$ is a solution to the equation $x^{2}+12x+c=0$, then $x=18$ satisfies this equation. Thus, $18^{2}+12(18)+c=0$ and so $324+216+c=0$ or $c=-540$. Therefore, the original equation becomes $x^{2}+12x-540=0$ or $(x-18)(x+30)=0$. Therefore, the other solution is $x=-30$.