Have they always been in control? Yes, for the entire existence of the US it has been legally capable of creating a US Dollar and of turning it into a fiat curr

Have they always been in control? Yes, for the entire existence of the US it has been legally capable of creating a US Dollar and of turning it into a fiat currency. However, for large swaths of the time the US has been around that has not been politically feasible, and I'm not sure it would have been economically feasible, fiat currencies seem to have taken a while to gain stability and acceptance. For a large period of time the US was on a gold standard and de facto did not have control over monetary policy. It doesn't matter much that they technically had power if they didn't use it.