"He got the job done on the  delegates," Spicer said. "He was involved with the campaign in just under five months. Paul was hired to count delegates, that’s why he was brought in ... he did his job.”
Here is a review of Manafort's time with the campaign:
Bringing him aboard
Manafort, 67, joined the Trump campaign on March 29, 2016, when he was named campaign convention manager for the mid-July festivities in Cleveland.
Manafort was "volunteering his considerable insight and expertise because of his belief that Mr. Trump is the right person for these difficult times," a news release from the Trump campaign said at the time.
"Paul Manafort, and the team I am building, bring the needed skill sets to ensure that the will of the Republican voters, not the Washington political establishment, determines who will be the nominee for the Republican Party."
Within weeks of his coming aboard, Manafort made clear his vision for the future of the campaign. At a meeting April 16, Manafort laid out a vision for the Trump campaign that included increasing spending, hiring more staffers and redefining their roles, sources told ABC News at the time.
Drama within the ranks
The drama reached a tipping point June 20 when Lewandowski was fired. Though Lewandowski didn't give a reason for his dismissal, he did say he was no longer running the show leading up to his dismissal.
"Paul Manafort has been in operational control of the campaign since April 7,” Lewandowski told The Associated Press on the day he was fired. “That's a fact.”
Lewandowski, who was hired by CNN as a political commentator days later, also spoke to the network immediately after he was fired, saying only positive things about Manafort.
"Paul helped us grow the campaign and we need to continue to grow to be successful. So everything that Paul did for the campaign is one less thing I had to do,” Lewandowski said. “I'm thankful for that.”
When asked whether his dismissal was the result of a power struggle between him and Manafort, Lewandowski said, "I don't think that's the case."
The move itself is common in many political campaigns, but it hadn't happened for the Trump campaign until Manafort was effectively at the helm.
Manafort was promoted to campaign chairman and chief strategist on May 19, effectively taking full command at that point.
Russia questions come up
Three days after the RNC wrapped up, Manafort addressed questions that had started to be raised about his and the campaign’s possible ties to Russia.
"No, there are not," Manafort said with a chuckle. "It's absurd and there's no basis to it."
He was asked if he any involvement in the change and Manafort said "I had none. In fact, I didn’t even hear about it until after our convention was over."
"It absolutely did not come from the campaign," he said.
The questions didn't die down over subsequent weeks, however. On Aug. 15, The New York Times reported that Manafort's name appears on a list of so-called black ledger accounts made by the toppled Ukrainian president amounting to $12.7 million from 2007 to 2012.
Manafort released a statement that day denying any wrongdoing.
"The simplest answer is the truth: I am a campaign professional,” he said. “It is well known that I do work in the United States and have done work on overseas campaigns as well. I have never received a single ‘off-the-books cash payment’ as falsely ‘reported’ by The New York Times, nor have I ever done work for the governments of Ukraine or Russia.”
The reports didn't stop there, and The Associated Press reported Aug. 18 that Manafort's firm had lobbied in the United States on behalf of the ruling Ukrainian political party even though Manafort did not disclose his work as a foreign agent, as mandated by federal law.
Manafort's departure from the campaign came the next day. On Aug. 19, Trump released a statement confirming Manafort's exit and praising his work on the campaign.
"This morning Paul Manafort offered, and I accepted, his resignation from the campaign,” Trump said in the statement. “I am very appreciative for his great work in helping to get us where we are today, and in particular his work guiding us through the delegate and convention process. Paul is a true professional and I wish him the greatest success.”
ABC News' John Santucci and Candace Smith contributed to this report.