Mackay told the board that he planned to retire effective January 1, 2011, to spend more time with his family, according to company spokeswoman Kris Charles.
We can't help being surprised by the news today, Stifel Nicolaus analyst Christopher Growe wrote in a note to clients, although he also noted that speculation as to Mackay's retirement preceded the announcement.
The board elected Bryant to become CEO on January 2, in accordance with its succession plan.
Bryant was actively involved in setting plans for 2011, so will probably not make any radical adjustments to the business, Growe said.
Mackay, 55, will work on the transition through March 31, the company said.
He is chairman of the Healthy Weight Commitment Foundation, a CEO-led coalition that has pledged to reduce the number of calories in the American diet.
Bryant, 45, will receive a base salary of $1 million, according to a filing with the U.S. Securities and Exchange Commission.
The company also confirmed its outlook for 2010 and 2011. It sees operating profit flat in 2010, and flat to down 2 percent in 2011.
Kellogg's shares were little changed at $49.53, up 3 cents, in late-morning trading on the New York Stock Exchange.
(Reporting by Helen Chernikoff; Editing by Lisa Von Ahn and Maureen Bavdek)