WHEN LUTHER W. Youngdahl came to Washington in 1951, we noted in this space that the only mystery about his appointment as a federal district judge was why he accepted it. Judge Youngdahl, who died Wednesday, had resigned from the Minnesota Supreme Court to run for governor and was in the middle of his third term in that office. He was the state's most popular political figure and widely regarded as a serious threat to the political future of its first-term senator, Hubert H. Humphrey. But he turned his back on all that to accept an appointment that Sen. Humphrey had persuaded President Truman to offer.

It turned out, when Judge Youngdahl talked about it years later, that there was no mystery. He had grown tired of political battles, had never really liked being an appellate judge, and wanted to finish his career trying cases in a courtroom. Try cases he did - vigorously and fairly - for almost two decades.

The most famous event of those years was his confrontation with the Department of Justice over the case of Owen Lattimore. After he had blocked the department's efforts to railroad Mr. Lattimore to jail for being a "communist sympathizer," its lawyers launched a monstrous attack on his personal integrity. The judge never flinched from doing what he thought was right, despite the heavy criticism - McCarthyism was in full sway just then - and in the long run both his integrity and his decisions were vindicated.

That episode brought out the tough and stern side that Judge Youngdahl had shown when he cracked down on organized crime in Minnesota. But there was another, compassionate side. We recall the day when his chambers became a juvenile courtroom because he believed the youth brought before him to be tried as an adult did not deserve such harsh treatment. It was the first, and perhaps the only, time a federal district judge had taken that step. But Judge Youngdahl was right again. The youth, whom he placed on probation, caused no more problems.

It was this mixture of gentle firmness and a belief in his fellow men that made Mr. Youngdahl a remarkable governor and judge. The judiciary, this city, and his home state of Minnesota, were all immeasurably richer for his presence.