"It's very hard to leave Lehigh," Gast said, "but it's an exceptional opportunity. Imperial is one of the world's great universities."
The news stunned staff on Lehigh's campus, still on what would normally be a quiet winter break. Classes resume Jan. 13.
Gast's departure comes as several other area universities are in the midst of presidential searches, including Pennsylvania State University, La Salle University, Bryn Mawr College, Cabrini College, and Community College of Philadelphia.
Gast, described by Imperial as its first leader from overseas, said the college called her. Imperial is restructuring its leadership, for the first time adopting the president/provost model used in the United States. It is also interested in the U.S. model of a president as chief fund-raiser, she said.
During her tenure at Lehigh, Gast oversaw completion of a $500 million fund-raising campaign and the launch of a second campaign that has brought in $225 million.
"The focus of a U.S. president on philanthropy and fund-raising is certainly a quality they were seeking," Gast said.
Gast became Lehigh's 13th president in 2006, also making history there as the first female president.
"Women leaders try to work very collaboratively and are consensus-oriented," she said. "I've really tried to use those skills in my leadership at Lehigh, and I will do the same at Imperial."
Born in Houston, Gast received her bachelor's degree from the University of Southern California and her master's and doctorate from Princeton. She was a professor at Stanford from 1985 to 2001. At MIT, she was vice president for research and associate provost.
Gast is on the board of directors of Chevron Corp. and a trustee of King Abdullah University of Science and Technology in Saudi Arabia.
During her tenure at Lehigh, she completed a strategic plan, doubled the percentage of international students, and expanded relations with the city of Bethlehem. She said she was exceptionally proud of learning opportunities students get beyond the classroom. In 2012, Lehigh started an international internship program with funding from former auto executive Lee Iacocca, an Allentown native.
Gast is in her second five-year term as president; she was reappointed in 2010. She is one of the region's highest-paid presidents. Her total compensation topped $1.1 million in 2011, placing her 35th in the country among the nation's private universities, according to the Chronicle of Higher Education.
"Alice has been a remarkable leader," Brad Eric Scheler, chair of the board of trustees, said in a statement. "With boundless energy, enthusiasm, wisdom, judgment and insight, Alice has advanced and best positioned Lehigh to be at the forefront of higher education in the 21st century."
A committee will be appointed to search for the next leader, Scheler said.
Penn State was poised to hire a New York medical college president in November until allegations surfaced that he had received unauthorized pay. Its search is continuing, with officials promising a replacement by June, when president Rodney Erickson is scheduled to depart.
Kim Cassidy continues to serve as interim president of Bryn Mawr as a search continues. Cabrini hopes to bring candidates for its presidency onto the campus early this year. La Salle also continues its search, with a selection expected this spring.