"I am not either of those people in that photo", Northam, a Democrat, told media gathered at the Executive Mansion, referring to the image of a person dressed in blackface and another in a Ku Klux Klan robe on his yearbook page.
"Yesterday I took responsibility for content that appeared on my page in the Eastern Virginia Medical School yearbook that was clearly racist and offensive".
Northam, 59, released a statement and a video in which he admitted to appearing in the photo, although he did not say which costume he wore. It's 2019, 35 years later; what Northam thought was in good taste when he was 25 says next to nothing, I think, about his beliefs and actions today, when he is 60.
Northam is calling state lawmakers Saturday to try and gain support so he can remain in office, the Democrat said.
Actress Alyssa Milano added to the #NorthamMustResign hashtag and said, "I can't believe we are having a debate on whether the guy in the blackface or the guy with the Klan hood is worse". He also repeatedly sidestepped questions about offensiveness of such an act, and refused again and again to resign despite the numerous calls for him to do so.
"We agree with the powerful words of our colleagues in the Virginia Legislative Black Caucus and believe that because of this photo the Governor has lost the confidence of the citizens he serves".
Mr. Northam, speaking in Richmond, cast himself as a martyr of his times, saying he won't resign because that would be the easy way out. "We've often wondered over the last 10 years or so why someone didn't dig this up sooner", said Joan Naidorf, a nonpracticing emergency-room physician who lives in Alexandria.
While she didn't directly call on Northam to resign, she said the photo was "also not reflective of someone who should be leading our Commonwealth" and called on the governor to "do what is best to allow our Commonwealth to heal going forward".
"And I said, 'Ralph, this is a day late and a dollar short. No matter the party affiliation, we cannot stand for such behavior, which is why the @NAACP is calling for the resignation of Virginia Governor".
Northam is the latest high-profile figure to run into trouble over blackface - the racist mimicking of African-Americans that dates back to the 19th century.
Calls for his resignation also poured in from Virginia's top Democrats and Republicans, as well as the Democratic presidential candidates Kamala Harris, Elizabeth Warren, Cory Booker, Kirsten Gillibrand and Julián Castro.
The Republicans join a growing chorus of elected officials in Virginia and elsewhere calling for Northam's resignation. The Virginia Democratic Party issued a statement demanding Northam's immediate resignation.
Added former US housing secretary Julian Castro: "This behaviour was racist and unconscionable".