It is a theorem of Mal'cev that all solvable subgroups of $GL(n,\mathbb Z)$ are polycyclic, and a theorem of Auslander that every polycyclic group is isomorphically embeddable in $GL(n,\mathbb Z)$. Auslander's theorem was later reproved by Swan purely algebraically adapting the proof of Ado's theorem.
A. I. Mal’cev, "On certain classes of infinite solvable groups", Mat. Sb. 28 (1951) 567–588; Amer. Math. Soc. Transl. (2) 2 (1956) 1–21
R.G. Swan, "Representations of polycyclic groups", Proc. Amer. Math. Soc, 1967
L. Auslander, "On a problem of Philip Hall", The Annals of Mathematics, 1967