Throughout his career he focused on U.S. policy toward East Asia, particularly China, Taiwan, Hong Kong, Japan and the Korean Peninsula.
He joined the State Department in 1972, and has held a number of senior posts relating to East Asian affairs.
In 2005, he pleaded guilty to unauthorized possession of secret documents, and to lying to the Federal Bureau of Investigation regarding his sexual relationship with a Taiwanese intelligence officer, Isabelle Cheng.
He denied, however, passing any confidential information to Cheng, and entered into a plea bargain arrangement — in exchange for prosecutors dropping espionage charges, he would co-operate fully in their investigations.
In January 2007, Keyser was sentenced for the original charges of possessing secret documents and lying to investigators — he served one year and one day in jail, and was fined $25,000.