NEW YORK — When Mayor Bill de Blasio acknowledged this month that his administration had kept secret that a former chief of staff was forced to resign after two women accused him of sexual harassment, the mayor said he was shocked and sickened by the aide’s conduct.

But it turns out that barely a month before he was hired by the de Blasio administration, the aide, Kevin O’Brien, had been fired from his previous job as a senior adviser at the Democratic Governors Association in Washington for similar reasons, the association confirmed Monday.

O’Brien had been a senior staff member for Gov. Steve Bullock of Montana, a centrist Democrat who has been mentioned as a possible 2020 presidential candidate. Bullock served a one-year term as chairman of the governors association in 2015, and he sent O’Brien to Washington to be his representative at the organization.

When O’Brien left the governors association in December 2015, Bullock was among those who knew the reason: A woman employed at the association had accused O’Brien of sexually harassing her, and an investigation had backed her up.

De Blasio’s press secretary, Eric F. Phillips, said Monday night that the mayor did not know of O’Brien’s dismissal from the governors association. “He was made aware of the prior situation for the first time today, as a result of your reporting,” Phillips said.

Phillips said that the Department of Investigation, as part of a standard background check, contacted both the governors association and an administrative office that deals with personnel for the state of Montana. He said that DOI “received confirmation of title and work dates and no adverse information” about O’Brien’s employment.

As part of the process, O’Brien also filled out two questionnaires that included queries about previous employment.

There was no indication whether Bullock had reached out to warn de Blasio about the harassment incident.

O’Brien first went to work for Bullock in Montana in 2009, when Bullock was attorney general, according to a résumé that O’Brien submitted to officials in New York. He was campaign manager during Bullock’s successful run for governor in 2012 and became deputy chief of staff to the governor in 2013.

O’Brien’s appointment to the governors association was announced in February 2015. Ten months later, he was let go after the association substantiated the harassment claim against O’Brien, according to a governors association official familiar with the case.

“When I was made aware of the complaint against Mr. O’Brien at the Democratic Governors Association, I fully agreed with the decision to end his employment,” Bullock said in a written statement provided to The New York Times.

In an apparent reference to the subsequent harassment case in New York, Bullock added: “It’s clear that was not enough to protect these women from what has proven to be an unacceptable pattern of behavior on his part. We all have a responsibility to do better and to put an end to sexual harassment, and I’m committed to doing my part.”

Bullock said that he was “deeply troubled” to learn of the New York accusation when it was reported by The Times earlier this month.

Yet it was clear that Bullock was aware that O’Brien had gone on to work for de Blasio and that they remained in contact: According to de Blasio’s schedule, he met with Bullock and O’Brien at Gracie Mansion on Oct. 15, 2017.

After losing his post at the governors association, it did not take O’Brien long to land a new job. To do so, he relied on his friendship with a top Democratic political strategist, Nicholas Baldick, who has worked closely with Bullock and de Blasio. In a pair of emails on Dec. 8, 2015, de Blasio and Baldick discussed offering a job to O’Brien in City Hall.

“He may be available now,” de Blasio wrote to Baldick in an email with the subject line “Kevin O’Brien.” “Pls discuss with me before you talk to him, but my thought is: We could slot him in as Deputy Chief of Staff and see how he does. Thoughts?”

Baldick replied: “Yes, not sure for that position. Will hold off talking to him, but he will likely call today.”

An earlier email revealed that Baldick, founder of the consulting group Hilltop Public Solutions, had previously tried to help O’Brien get a job in the de Blasio administration, shortly after de Blasio took office in 2014.

The emails were among thousands of pages of correspondence between the de Blasio administration and a group of outside advisers that were made public after news organizations sued City Hall to obtain the material.

When reached for comment Monday, O’Brien, who is married and in his 30s, issued a statement nearly identical to one that he released after the circumstances of his departure from City Hall were disclosed, blaming alcohol abuse for making “horrible decisions.”

“There’s no excuse for what I’ve done. I’m embarrassed and ashamed,” the statement read. “No one deserves to be treated that way. I’ve apologized to the people I’ve hurt and will continue to do so because I am truly sorry.”

O’Brien started work at City Hall, as deputy chief of staff, on Jan. 24, 2016, with an annual salary of $175,000, according to city records. He was promoted to acting chief of staff in early 2017 and his salary was increased to $220,652.

He served in that capacity until the end of that year, following de Blasio’s re-election, when he became a senior adviser to the mayor. In announcing the change, de Blasio praised O’Brien for doing an “outstanding job.”

In February 2018, two women accused O’Brien of sexual harassment and he was forced to quit City Hall, although he remained on the payroll until late March. City Hall made no announcement about O’Brien’s departure or the reason for it.

Once again, Baldick came to O’Brien’s rescue, hiring him in April 2018, as a principal in Hilltop’s New York office.

Hilltop fired O’Brien this month after The Times reported about the sexual harassment case against O’Brien at City Hall.

Baldick told The Times on Monday night that he “had no idea” that O’Brien had been fired from the governors association because of a sexual harassment claim.

“I was shocked and disappointed to learn about this side of Kevin,” he said. “I never would have recommended him, and I certainly wouldn’t have hired him to work at my firm.”

De Blasio said during a Jan. 17 news conference that he had not known until recently that O’Brien had gone to work at Hilltop; he expressed surprise that Hilltop had not contacted City Hall to check O’Brien’s references before giving him a job. He defended the decision to allow O’Brien to resign, rather than firing him, and to keep the reason for his exit secret, saying that it was intended to protect the women who made the accusations against him.

“The misconduct in this case literally makes me sick to my stomach,” de Blasio said at the time.

Had Hilltop or any other prospective employer contacted City Hall before hiring O’Brien, the mayor said, the company would have been told that he “resigned in lieu of termination,” which de Blasio said would have served as a warning.

“Any employer who did a routine reference check would have been given a red flag sufficient to act on,” de Blasio said at the news conference.

“I would also say it is a rare employer who doesn’t do a reference check on an employee. So I don’t think you should undercount that that red flag would have been perfectly, abundantly clear.”

In a separate statement, Bullock’s office said that “no incidents of sexual harassment by O’Brien were reported to the governor or his chiefs of staff during O’Brien’s time of employment at the governor’s office or the attorney general’s office.”

This article originally appeared in The New York Times.