A former Trump campaign official described their relationship as one centered on “mutual respect” and described Giuliani as a “sounding board” for the president, predicting the two would "align their effectiveness" in their attorney-client relationship.
“Rudy is great. He has been my friend for a long time and wants to get this matter quickly resolved for the good of the country,” Trump said in a statement released by his attorneys last week.
Giuliani called it “an honor to be part of such an important legal team” in a statement after the announcement, adding that he looks forward to working with the president and his current team of lawyers.
Giuliani was also floated during the presidential transition as a possible cabinet nominee for a variety of roles, ranging from secretary of state to attorney general. In the end, the administration never brought him in for any cabinet-level position. The transition team said in a statement at the time that Giuliani asked to be removed from consideration for an administration position during a meeting with President-elect Trump on November 29, 2016.
“Rudy would have been an outstanding member of the Cabinet in several roles, but I fully respect and understand his reasons for remaining in the private sector,” Trump said at the time.
"There are real questions that are raised," Laurie Levenson, a professor at Loyola Law School told ABC News. "Giuliani himself was involved in the campaign and had business ties with people who we think are subjects or targets of the [Mueller] investigation. At some point, I think it's going to be difficult for him to continue on as counsel. The special counsel may want to know -- what does he know?" Levenson said it's hard to know whether this ripens into a full blown conflict, because we don't know all angles Mueller's investigation.
"Giuliani cannot discuss with Mueller any matter in which Giuliani had a client, most obviously his representation of Zarrab," said Stephen Gillers, a professor at NYU's law school.
Giuliani was also touted in a press release for TriGlobal Strategic Ventures, a business consulting company tied to Russia.
While ABC News has not reported any connection between Mueller and Giuliani's business ties, legal experts say any potential ties he may have to the investigation do not effect his role on Trump's legal team -- at least for now.
"Giuliani may be a witness in the Mueller investigation so he is in the somewhat strange position of advocating for Trump while also a potential source of information for Mueller," said Gillers. "That information would most likely come from his unofficial role in the campaign. But while this dual status can be awkward, it does not prevent Giuliani from representing Trump, at least not based on what we know."
However, Trump's legal team is adamant that Giuliani's past legal work and business ventures present no legal conflicts.
Among the many matters related to the 2016 election that the Department of Justice Inspector General is looking into is the question of whether information was inappropriately shared with Giuliani ahead of the election, according to officials familiar with the matter.
"I do think that all of these revelations about Hillary Clinton finally are beginning to have an impact," Giuliani said on Fox News in October 2016. "[Trump's] got a surprise or two that you're going to hear about in the next two days." It's not clear if this is what the IG is probing.
But, now with negotiations ongoing with the special counsel team investigating Russia meddling in the 2016 election, Giuliani, 73, has been brought back into the fold at a time when multiple former associates close to the president have faced serious legal peril as a result of the Russia investigation. Giuliani is expected to go head to head with Mueller, despite their own professional history of working together dating back to Mueller’s time as FBI Director in the wake of the World Trade Center attacks.
Giuliani has spent the majority of his career in domestic politics but is also the former U.S. attorney for the Southern District of New York, the very same federal prosecutor’s office that, years later, ordered the FBI raid on Trump’s personal lawyer, Michael Cohen.
For six years, Giuliani earned a reputation as a tough U.S. attorney fighting drugs, violence and organized crime in the city. He prosecuted a number of high-profile cases, including that of Wall Street trader Ivan Boesky, and indicted 11 heads of organized crime in the Mafia commission trial.
Despite a failed run for New York mayor in 1989 (Trump co-chaired his first fundraiser that year), Giuliani ran again as the Republican candidate in 1993 and won. He went on to serve two terms and cement his place in the city’s history.
As mayor, Giuliani worked to reform the New York City welfare system by requiring able-bodied people to work or perform community service to receive payments. The welfare-to-work initiative caused welfare rolls to drop more than 50 percent during his eight years in office.
With his appointed police commissioner, Bill Bratton, Giuliani aggressively tackled the city’s crime problem. By the time he left office, the violent crime rate had fallen 56 percent, according to PolitiFact. While robberies plummeted by 67 percent and murders fell by nearly two-thirds, some groups such as the NAACP and the ACLU have criticized the police tactics, such as stop and frisk, and raised concerns about racial profiling.
"The city may have been safer after his stay there, but he really polluted the city along racial lines. He split the city up really bad in a nasty way," Rep. Adriano Espaillat, D-NY, told ABC News.
"His record in city hall shows clearly that he was one of the most divisive mayors in the city, he divided the city along ethnic and racial lines, that was clear," said Rep. Meeks.
Some critics also point to data from the U.S. Justice Department that show New York City crime rates began to drop in the years before his mayoralty, saying that declines were part of a broader trend also seen in Chicago, San Diego, Miami and other cities.
When Giuliani took a bigger role in Trump’s campaign, critics and former mayoral staffers said he had changed, becoming more willing to fall in line with Trump and more combative.
Giuliani was a top surrogate for Trump during the 2016 campaign, often seen by the candidate’s side at multiple campaign rallies.
“I’m sick and tired of the defamation of Donald Trump by the media and the Clinton campaign,” Giuliani said at the 2016 Republican convention about the candidate’s critics. “I am sick and tired of it! This is a good man!”
In 2000, while still serving as mayor, Giuliani ran against Hillary Clinton in New York for the U.S. Senate. A diagnosis of prostate cancer led him to withdraw from the race.
During his multiple TV appearances in the 2016 campaign in support of Trump, Giuliani cultivated a reputation for his fiery rhetoric aimed at Clinton that would at times devolve into personal or misleading attacks.
In addition to Giuliani promoting conspiracy theories that Clinton was severely ailing in her physical health, he also claimed in a campaign rally that Clinton was not present in New York City after the 9/11 attacks, despite a picture that showed the two walking together side-by-side down a street near Ground Zero. Giuliani later corrected his statement and apologized to Clinton.
Giuliani’s hire onto the Trump legal team comes as the president has escalated his attacks against special counsel Robert Mueller.
For the last several months, Trump lawyers have been in active negotiations with Mueller's team, working toward a potential interview of the president that would include either a face-to-face interview with parameters, a written questionnaire or some mix of both, sources have told ABC News.
The last reported meeting the Trump team had with the special counsel's office was on April 9, the same day that FBI agents in New York raided the home, office, and hotel of Trump’s personal attorney, Michael Cohen. Sources told ABC News that since the Cohen raid, the president has since been “less inclined” to sit down for an interview with Mueller’s team.
Giuliani enters the arena after the president’s lead attorney John Dowd abruptly resigned in March. Sources told ABC News at the time that Dowd resigned in part because he felt the president was not taking his advice.
But time will tell if Giuliani's previous professional relationship with Mueller bodes well for Trump.
"I think that just because you know Mueller does not mean anything," Arzt said. "You gotta come across with some substance."
Benjamin Siegel and Aaron Katersky contributed to this report.