President Bush's top aide Dan Bartlett announced his resignation Friday, ending a lengthy career at the president's side to take a job in the private sector, FOX News has confirmed.

Bartlett's decision to leave in early July came on his 36th birthday. He has served Bush for about 14 years, from the Texas governor's mansion to the White House.

Bush thanked Bartlett for his dedication and friendship.

"His contribution has been immeasurable. I value his judgment and I treasure his friendship," the president said in a statement. "Since coming to work for me 14 years ago as I prepared to run for governor, Dan has become a husband and a father. I understand his decision to make his young family his first priority."

Bartlett, who has three children under the age of 4, leaves for a job that will allow him to spend more time with his young family.

With twin 3-year-old boys and another son born in January, Bartlett said it was time to pursue a new chapter of his life and "reacquaint myself with my family." His wife, Allyson, had joked that they should name their newborn "Exit Strategy."

Click here to read Dan Bartlett's biography.

Serving as counselor to the president, Bartlett also worked in Bush's first campaign as governor of Texas and two subsequent presidential races.

Before joining Bush, Bartlett worked for presidential adviser Karl Rove's company back in Texas, an Austin-based political consulting firm. Bartlett graduated from the University of Texas with a bachelor's degree in political science.

Bush trusts Bartlett, who is known as someone in the inner circle who has the president's ear. Bartlett is able to deliver bad news to the president or tease him about wearing a brown suit nicknamed 'Big Brown.'

"He can talk to the president in a candid way, in sort of a family way, that almost nobody else can," White House chief of staff Joshua Bolten told The Associated Press. "He can talk to him about Big Brown, he can joke with him. He's got the Texas roots that make it possible for them to talk about characters in Texas politics or Longhorn football or Texas Rangers' baseball. He's been a good friend of the president as well as a counselor."

Before Bartlett, Defense Secretary Donald Rumsfeld's resignation last November marks the last major departure from the Bush administration.

FOX News' Mike Emanuel and The Associated Press contributed to this report.