The Reformed theologians and churches have held, taught, and confessed the existence of natural law since the earliest days of the Reformed Reformation in the 1520s. If all one knows, however, of Reformed theology is 20th century (or 21st-century) Reformed theology, one might not know how important natural law has been to Reformed theology and ethics. For the Reformed, as for Luther, Melanchthon, and the rest of the magisterial Reformers and their orthodox successors, the moral law, the Decalogue (the Ten Commandments), is substantially identical to the natural law.
In ethics, when the Reformed thought about civil government, they did not think about the Mosaic judicial laws, which they confessed and taught to have been fulfilled by Christ and expired with the death of Christ. Rather, they thought of the natural law. The general equity of the Mosaic judicial laws was said to remain in force only insofar as they were identical to the natural law.
The natural law was important to the Reformed for their covenant theology. The covenant of works, instituted before the fall, was said to be an expression of the natural law. The Sinaitic law was said to be a republication of the covenant of works or the law of nature.
The Reformed doctrine of natural law also reflected the Reformed conviction that grace restores nature. First, as catholic, ecumenical Christians, i.e., anti-Gnostic, the Reformed affirmed the goodness of creation per se. The fall was said to have corrupted nature but not to have obliterated it. Grace, in traditional Reformed theology, was not said to perfect nature (as in Thomas) but nor to obliterate it (as with the Anabaptists) but to restore it in redeemed sinners.
Here are some resources on the Reformed use of natural law: Resources On Natural Law