McCormick Lelyveld, 32, who joined Obama's staff in April 2007, said she will pursue a job in the private sector.
McCormick Lelyveld is a graduate of the Latin School of Chicago, St. Ignatius College Prep and Georgetown University. She interned for first lady Hillary Rodham Clinton during college and worked on Massachusetts Sen. John Kerry's unsuccessful presidential bid in 2004.
She said she joined the presidential campaign after hearing an early speech by Obama. "She seemed so incredible," Lelyveld said, "and someone I could really learn from and help introduce to the country."
McCormick Lelyveld helped shape the first lady's image, traveling with her to most of the states and 13 foreign countries.
"Katie has been with me since the very beginning and I've relied on her intelligence, grace, quick wit, humor and friendship through the campaign and at the White House," Obama said. "I will always be grateful for her hard work and dedication. She will be missed, but I know she will do great things in Chicago."
McCormick Lelyveld said she has ruled out joining the re-election campaign or taking a government job. "It has been an honor for me to support a family I believed in and have come to adore," she said. "But I'm incredibly excited to get closer to my family and dear friends and start the next chapters."
Her father, physician Steven Lelyveld, is with the University of Chicago Medical Center, and her mother, Betsy McCormick, formerly directed the medical library at Children's Memorial Hospital.
McCormick Lelyveld is the most recent Chicagoan to depart the White House, coming after Rahm Emanuel, now mayor-elect; political guru David Axelrod, who will steer the 2012 campaign; Susan Sher, formerly the first lady's top aide; and Desiree Rogers, her former social secretary.
McCormick Lelyveld's last day in the East Wing is May 27.