William Thorsell, seen in 2007 speaking about the ROM's Daniel Libeskind-designed addition: the Michael Lee-Chin Crystal. (Adrian Wyle/Canadian Press)After nearly a decade of heading up the Royal Ontario Museum and leading a bold expansion project designed by Daniel Libeskind, William Thorsell plans to step down as the ROM's director and CEO.

Thorsell will retire in August 2010, 10 years from the start of his tenure at the Toronto museum, ROM officials said Thursday.

"I believe there is a season to things," Thorsell said in a statement.

"The museum will benefit from new skills and passions in its leadership as the agenda evolves. I have enjoyed the luxury of a wonderful assignment supported by generous boards, remarkable donors, talented staff, volunteers and governments."

Thorsell joined the ROM in August 2000 after a decade as editor-in-chief of the Globe and Mail newspaper. Before that, he was a journalist and editor for the Globe, as well as the Edmonton Journal.

He had also worked as an administrator at the Woodrow Wilson School of Public & International Affairs at Princeton University and on the World Expos in Montreal in 1967 and in Osaka in 1970.

The ROM launched its massive renovation, restoration and expansion project in 2001. The audacious and controversial new addition, the Libeskind-designed Michael Lee-Chin Crystal, opened in 2007.