Rachel Noerdlinger, center, in January. Behind her, from left, Mayor Bill de Blasio, Chirlane McCray and the Rev. Al Sharpton. Peter Foley for the Wall Street Journal

A close adviser to New York City Mayor Bill de Blasio and his wife won’t face disciplinary action after an investigation found that the aide didn’t disclose during a background check that she lived with her boyfriend, officials said Friday.

The city Department of Investigation has delivered to City Hall the final results of its review of a background form filed by the aide, Rachel Noerdlinger, the mayor’s spokesman said Friday.

Ms. Noerdlinger and her live-in boyfriend, Hassaun McFarlan, have been scrutinized after the disclosure of his criminal history.

Ms. Noerdlinger is chief of staff to Mr. de Blasio’s wife, Chirlane McCray, and had to fill out a lengthy questionnaire before taking her post earlier this year.

The city Department of Investigation found that Ms. Noerdlinger didn’t reveal Mr. McFarlan lived with her in Edgewater, N.J., but that she didn’t intend to deceive the mayor or City Hall, said a de Blasio spokesman, Phil Walzak.

Mr. Walzak said City Hall took the matter seriously and would note the omission in Ms. Noerdlinger’s personnel file. He said city officials believed “no further punitive action is necessary.”

Ms. Noerdlinger and Mr. McFarlan declined to comment through City Hall.

Ms. Noerdlinger’s ties to Mr. McFarlan were questioned after DNAInfo.com reported on Mr. McFarlan’s 1993 manslaughter conviction in a fatal shooting and other alleged crimes. The website also reported on Facebook posts in which Mr. McFarlan made derogatory comments about police officers.

Mr. de Blasio hired Ms. Noerdlinger in January from thestaff of the Rev. Al Sharpton, where she served as the civil-rights activist’s spokeswoman. She makes $170,000 a year working for Ms. McCray.

The mayor’s office had worked to contain the controversy over Mr. McFarlan with limited success. The city’s police unions—at odds with the mayor over the level of his support for officers—have decried Ms. Noerdlinger’s relationship with Mr. McFarlan, calling it a conflict for the mayor to have an adviser whose loved one harbors anti-police viewpoints.

Mr. de Blasio previously dismissed any criticism of Ms. Noerdlinger in connection with her relationship with Mr. McFarlan.

“You don’t fire people because of something their boyfriend said. That’s ludicrous,” he said. “I have absolute faith in her.”

“We don’t care what someone’s boyfriend said,” the mayor added, referring to Mr. McFarlan’s online commentary. “We care what the public servant is about, and she is very dedicated to the work of bringing police and community together. And that’s what we care about.”

Earlier Friday, Mr. Walzak issued a statement praising Ms. Noerdlinger for going to “work every day committed to serving the people of our city across the five boroughs on the critical issues that matter.”

The questionnaire at issue was part of the city Department of Investigation’s annual background investigation of new city employees or those promoted to management positions.

Officials who must fill out these background checks include all managerial positions, individuals earning more than $80,000 annually, all individuals directly involved in city contracts and employees who work on the city’s sensitive computer programs. About 2,000 such investigations are performed each year, according to the Department of Investigation.

The form specifically asks officials to list the names of any people who are residing in any residence that the official maintains, whether that person is related or not.

Michael Palladino, president of the Detectives’ Endowment Association, said it is a “serious situation” if Ms. Noerdlinger didn’t accurately fill out her background form.

“People who are hired in sensitive positions in City Hall must be held to a higher standard,” he said. “Obviously a mistake has been made hiring her and the de Blasios should cut their losses and move on to a replacement.”

Patrick Lynch, president of the Patrolmen’s Benevolent Association, issued a statement raising questions about her companion’s criminal history and his posts on social media.

“The standards that apply to hiring police officers should apply equally to hiring high ranking, influential staff members,” Mr. Lynch said. “If it is found that she committed a lie of omission during the investigation, then she should be fired.”

Ms. Noerdlinger symbolized Mr. de Blasio’s close relationship with Mr. Sharpton, a break from past mayors who kept the reverend at arm’s length. On Friday, Mr. Sharpton said the recent media coverage of Ms. Noerdlinger “smells to me like a witch hunt.”

“Since when is a woman responsible for her boyfriend’s behavior 15 years ago or his comments,” Mr. Sharpton said. “This is ridiculous to me.”

He described Ms. Noerdlinger as a “moderate” on his staff.

“She was always the one saying, ‘We’ve got to work with police,’” Mr. Sharpton said. “Of all people Rachel would not be the one we would consider anti-cop. She was always the one who was coolheaded and always the one that was trying to keep us doing things more mainstream.”

Write to Michael Howard Saul at michael.saul@wsj.com