William Aitcheson, a priest for the Roman Catholic Diocese of Arlington, Virginia, announced his leave of absence in an editorial for the Arlington Catholic Herald.
“When I think back on burning crosses, a threatening letter, and so on, I feel as though I am speaking of somebody else,” he wrote. "It’s hard to believe that was me.”
The 62-year-old said he had joined the Klan as an “impressionable young man” who was “in no way practising my faith”.
According to the Washington Post, Mr Aitcheson was a leader of the Robert E Lee Lodge of the Maryland Knights of the KKK in the 1970s.
The group, which boasted about a dozen members, was allegedly planning to bomb the homes of black people and the offices of the NAACP in Prince George's, Maryland.
Police officers who searched Mr Aitcheson’s home at the time found nine pounds of black powder, weapons and bomb parts.
“While 40 years have passed, I must say this: I’m sorry,” Mr Aitcheson wrote. “To anyone who has been subjected to racism or bigotry, I am sorry. I have no excuse, but I hope you will forgive me.”
Mr Aitcheson credited his change of heart to “a lot of soul searching" and a return to Catholicism. He was ordained in 1988 and served at churches in Nevada, Maryland, and Virginia, according to church officials. He most recently worked as an assistant to the pastor at St Leo the Great in Fairfax City.
“We must condemn, at every opportunity, the hatred and vile beliefs of the KKK and other white supremacist organisations,” he wrote on Monday. “What they believe directly contradicts what we believe as Americans and what we, as Catholics, hold dear.”
The priest’s admission comes on the heels of a white supremacist rally in Charlottesville, Virginia, where members of the KKK and other hate groups rallied to protest the removal of a Confederate statue. Three people were killed and dozens were injured in connection with the event.
Mr Aitcheson said the rally reminded him of his time in the KKK – a time he would have preferred to forget. But he warned readers against erasing past mistakes from their memories.
“[W]hile I firmly believe God forgave me — as he forgives anyone who repents and asks for forgiveness — forgetting what I did would be a mistake,” he wrote. “Those mistakes have emboldened me in my journey to follow the God who yearns to give us his grace and redemption.”