In the summer of 2015, President Barack Obama made his first large batch of commutations of the sentences of people who had spent years in prison for relatively minor drug offenses. One of them was Norman O'Neal Brown, a Maryland man who had spent half of his life in prison after selling crack cocaine to an undercover agent. Without Mr. Obama's intervention, the draconian mandatory minimum sentences in place in the early 1990s would have kept him behind bars for the rest of his life.

Instead, Mr. Brown, 49, is splitting time this holiday season between his sister, with whom he has lived in Waldorf since leaving prison, and his serious girlfriend, Jennifer, who lives in Washington. Returning to life outside of prison has been a big adjustment — everything from grocery shopping to driving to navigating personal relationships is hard for someone whose life was effectively shut off at age 22. But Mr. Brown is an extraordinary person; he was a leader behind bars in supporting other prisoners, and he continues that kind of work today. His day job is at Project New Opportunities, an organization that helps ex-offenders adjust to life outside; he was recently promoted to deputy project director. Four nights a week, he works for another organization that helps juvenile offenders.

In another sense, though, Mr. Brown isn't extraordinary at all; he is one of tens of thousands of young men, disproportionately African-Americans, who were thrown away by society decades ago in the mania of the war on drugs. And to what end? Has the epidemic of drug abuse abated because of the lengthy prison terms we imposed? Are our streets free of violence?

President Obama has long talked about the wasteful injustice of these policies, but it has only been toward the end of his term that he has used the singular power of his office to do something about it. In 2014, he invited applications for commutations from prisoners like Mr. Brown — people who had already served substantial time for non-violent offenses, who had records of good behavior in prison and who would not have received such lengthy sentences under current law. Thousands poured in. Mr. Brown was among a batch of 46 inmates Mr. Obama set free in July 2015, and since then he has rapidly amassed a record of clemency that dwarfs that of his predecessors. The 153 acts of clemency Mr. Obama approved last week bring his total to 1,176; additionally he has offered 148 full pardons. He has used his powers of clemency more than any president in history.

But Mr. Obama can't stop there. At least 12,000 applications are still pending, and even they don't begin to cover the number of people whose continued incarceration for drug crimes serves no purpose. It is not keeping us safer, it is not dissuading others from committing crimes, and it exacts punishment far out of proportion to the seriousness of the offense.

That is a reality that has increasingly been understood across the political spectrum — from the Koch brothers to the Open Society Institute — yet it is one that has not penetrated the consciousness of President-elect Donald Trump or key members of his Cabinet. Mr. Trump, who ominously intoned the phrase “law and order” from the campaign trail and endorsed discredited tactics like stop-and-frisk and mass incarceration, falsely claimed in a speech to the NRA that Mr. Obama “is even releasing violent criminals from the jails, including drug dealers, and those with gun crimes. And they're being let go by the thousands.” Meanwhile, Mr. Trump's nominee for attorney general, Sen. Jeff Sessions, has argued that Mr. Obama's actions violated the intent of the framers of the Constitution. For those deserving of commutations, it's now or never.

Mr. Brown says he and some others who received clemency had the chance to have lunch with Mr. Obama last spring, and the president asked them what he could do to improve the criminal justice system. “We said we thought he's only one man, and what he has done has been remarkable,” Mr. Brown said. “We said we hoped he would change some of these mandatory minimums. ... The time should fit the crime.”

There's little chance Mr. Obama can convince Congress to do that in the last month of his presidency. But he has the power to right injustices one by one. He should use it.