Pei soon continued his studies at Harvard University's Graduate School of Design, where he had the opportunity to study with German architect and founder of the Bauhaus design movement Walter Gropius. During World War II, Pei took a break from his education to work for the National Defense Research Committee. In 1944, he returned to Harvard and earned his master's degree in architecture two years later. Around this time, Pei also worked an assistant professor at the university.
In 1948, Pei joined New York-based architectural firm Webb & Knapp, Inc., as its director of architecture. In 1955 he left to start his own firm, I. M. Pei & Associates (now known as Pei Cobb Freed & Partners). One of his first major projects was the Mile High Center in Denver, Colorado. Pei also devised several urban renewal plans for areas of Washington, D.C., Boston and Philadelphia around this time.
In the years following the death of President John F. Kennedy, Pei met with his widow, Jacqueline Kennedy Onassis, on the designs for his presidential library. The project, built in Dorchester, Massachusetts, met several challenges over the years, including a change in location. Completed in 1979, the library is a nine-story modern structure that features glass and concrete. Pei also designed a later addition to the site.
Following the dedication of the Kennedy library, Pei continued to create wondrous buildings around the world, including the west wing of the Museum of Fine Arts in Boston (1980) and the Fragrant Hill Hotel in China (1983). In 1983, he was awarded the Pritzker Architecture Prize for his contributions to his field. In their official announcement, the committee recognized his ability to "draw together disparate people and disciplines to create an harmonious environment." Pei used his prize money to create a scholarship for Chinese students to study architecture in the United States.
During this time, Pei also began work on revitalizing Paris's Louvre museum. The new, and controversial, entrance he created for the world-famous structure has since become one of the most iconic representations of his work. Pei had visitors descend into the museum through a large glass pyramid, which took them to a new level below the existing courtyard.
Pei continued to design impressive buildings during the 1990s, including the United States Holocaust Memorial Museum in Washington, D.C., and the Rock and Roll Hall of Fame in Cleveland, Ohio.