The attack occurred at around 7:40 a.m. when Lippert was attending a lecture meeting at the Sejong Hall of the Sejong Center for the Performing Arts in central Seoul, the reports said.
Lippert, bleeding, was immediately moved to a nearby hospital for treatment. He received knife wounds to his right cheek and left wrist, officials at Severance Hospital at Yonsei University said.
The suspect, identified as Kim Ki-jong, was arrested at the scene.
Kim, who heads a liberal group, has voiced strong objections to annual South Korea-U.S. military drills, sources said.
In 2010, the suspect received a suspended two-year prison term for throwing a piece of concrete at then-Japanese Ambassador Toshinori Shigeie during his speech in Seoul.
The breakfast meeting was organized by the Korean Council for Reconciliation and Cooperation.