Six months into his presidency, Donald Trump’s detractors portray him as a do-nothing president with no big wins on issues such as health care, taxes and infrastructure.

That may be true if the benchmark is legislation, but that is an incomplete benchmark. To gauge a president’s impact you have to go beyond the laws he signs to the vast authority he wields through departments and agencies that apply the law. On that score, Mr. Trump is on track to do a lot. On finance, the internet, immigration and drugs, to name just a few...