The United States condemned the “act of violence” which left the ambassador bleeding profusely as he was rushed to hospital, and said that President Barack Obama had spoken with him.
Witnesses described how a man with a blade concealed in his right hand had lunged across a table at Lippert at a breakfast function at the Sejong Cultural Institute in central Seoul.
Video footage showed the ambassador being escorted out of the building holding one hand to his bleeding right cheek, and his other hand smeared with blood with an apparent wound to the wrist.
Lippert, 42, was bundled into a police car and rushed to hospital, where a US embassy spokesman said he was in a “stable” condition.
Police identified the attacker as Kim Ki-Jong, 55, and said he was a left-wing nationalist with a previous conviction for hurling a stone at the then Japanese ambassador to Seoul in 2010.
District police chief Yoon Myung-Soon said Kim had slashed the ambassador with a 25-centimetre (10-inch) paring knife.
“We have detained him and are investigating the cause of the attack and other circumstances,” he said.