This post has been updated. 

Donald Trump's national field director Stuart Jolly resigned on Monday amid major changes to the campaign's top leadership and a swift transfer of power away from a small group of devoted staffers who have been with Trump since the beginning to a larger team of more experienced operatives.

Jolly, 52, said in an interview Monday night that he decided the time had come for him to leave the campaign, and he's not angry about doing so. He resigned in a letter addressed to "Mr. Trump" on Monday that thanked the Republican front-runner for hiring him, even though he had never worked on a national campaign before.

"As your National Field Director, my guidance to the states’ staff and strike teams was to always to focus, keep moving, grow the base, love on your volunteers, and take care of your people," Jolly wrote in the letter, which he shared with The Washington Post. "We did these simple things, we won, and we continued to win because we followed a winning formula. You know my advice to you, and my advice has not changed since the beginning. My hope is that you will continue to listen to those who have propelled you to victory."

Jolly is a longtime friend of Trump's campaign manager, Corey Lewandowski, and the two previously worked together at Americans for Prosperity, an advocacy group that is backed by a network of conservative donors organized by brothers David and Charles Koch. Jolly said that part of the reason he joined the campaign was to help Lewandowski. While Lewandowski has been known to be tough on his staff, sometimes yelling or speaking harshly, Jolly was considered his friendly, soothing counterpart. The two oversaw win after win in early states, even as they spent a small fraction of the amount of money spent by other campaigns. Jolly said Monday that he always had as much funding as he needed.

As Trump's chances of locking up the GOP nomination before the convention in July have faded, Lewandowski has seen his role in the campaign lessened. Trump has brought in a group of aides with decades of national political experience, including Paul Manafort, a longtime strategist who was recently brought on to oversee Trump's delegate and convention strategy. Manafort has quickly assumed the role of Trump's top adviser and reports directly to him.

Jolly -- whose resignation was first reported by Politico -- said in an interview that he recently learned that he would no longer report to Lewandowski and would instead report to newly hired national political director Rick Wiley, who previously managed Wisconsin Gov. Scott Walker's campaign.

"That wasn't going to happen," Jolly said of the change in leadership. "I just decided not to go that route and let the new team have a shot at it. We have a great group of folks in place on the ground, and they're kicking butt and doing great work. We had 22 successful campaigns -- our guys on the ground just did a fantastic job."

Jolly made clear in his letter to Trump that his resignation is not because of the candidate, Lewandowski or the field staff.

"It was time for me to go," Jolly said in an interview. "I would rather go out on top than, you know, go after a loss or something. I went out on my own -- I wasn't pushed out or forced out. I wasn't asked to leave, quite the opposite. It's been a good ride. I would do it all again."

On Monday Trump also hired one of the GOP's top legal minds on convention and delegate matters. William McGinley, a veteran Republican elections attorney, will advise the campaign on the arcane process of selecting delegates and the convention rules process, a person familiar with the move said late Monday. News of the hire was first reported by Politico.

McGinley is a partner at Jones Day, the same firm that employs Don McGahn, Trump's top campaign attorney. While better known in Washington for representing members of Congress facing ethics issues, he advised the 2012 GOP convention rules committee and once advised the Republican National Committee and National Republican Senatorial Committee on ethics, ballot and campaign finance issues. His background will lend much-needed knowledge to a campaign still struggling to master the trickiest elements of presidential politics.

What Donald Trump is doing on the campaign trail

U.S. Republican presidential nominee Donald Trump speaks at a campaign event at Trump Doral golf course in Miami, Florida, U.S. July 27, 2016. REUTERS/Carlo Allegri (Carlo Allegri/Reuters)