Herring's admission follows a chorus of calls for Northam to resign over a racist photo in his 1984 medical school yearbook, depicting a person in blackface makeup standing beside another person garbed in white robes of the white supremacist group the Ku Klux Klan.
"I am not either of the people in that photo", he said with his wife by his side.
Meanwhile, Northam's political heir apparent, Lieutenant Governor Justin Fairfax, 39, confronted a potential scandal of his own.
FILE PHOTO: Virginia Attorney General Mark Herring arrives at an election night rally for Governor Ralph Northam and Lt. Governor Justin Fairfax on the campus of George Mason University in Fairfax, Virginia, November 7, 2017.
"Democrats at the top are killing the Great State of Virginia", Trump tweeted Thursday morning.
"Mr Fairfax has tried to brand me as a liar to a national audience, in service to his political ambitions, and has threatened litigation", she said.
Asked earlier this week about the allegations against Fairfax, a series of high-profile Democrats reiterated their call for Northam to resign but wouldn't engage on that possibility in regard to Fairfax.
Kirk Cox, 61, the Republican speaker of the Virginia House of Delegates, is third in the state's constitutional line of succession. "But it doesn't change the fact that Northam needs to resign".
In his statement, Mr Herring said he and two friends dressed up to look like rappers they listened to, including Kurtis Blow, admitting: "It sounds ridiculous even now writing it". NBC host Megyn Kelly had her show canceled and was ousted from the network past year, after she suggested that wearing blackface is acceptable as part of a costume.
"That conduct clearly shows that, as a young man, I had a callous and inexcusable lack of awareness and insensitivity to the pain my behavior could inflict on others", wrote Herring.
Then as videos and pictures of college students wearing blackface popped up - leading universities to condemn the images and take disciplinary action. Later in his statement, he admitted: "That same year, I did participate in a dance contest in San Antonio in which I darkened my face as part of a Michael Jackson costume".
That radio interview is what reportedly motivated a medical school classmate of Northam's to tip the media off about the yearbook photo, triggering the current avalanche of recrimination.
But he also said: "This conduct is in no way reflective of the man I have become in the almost 40 years since".