Kim Ki-jong, 56, attacked Mark Lippert at a breakfast function in a Seoul hotel on March 5.
The US ambassador in South Korea needed 80 stitches in his face and hand and was left scarred.
Kim Ki-jong, a known Korean nationalist, had made multiple visits to North Korea.
He said he was protesting against joint South Korean-US military drills but did not intend to kill Mark Lippert.
The prosecution, however, argued the force of the attack was so great that it might well have been fatal. They had sought a 15-year prison term.
Kim Ki-jong was also convicted of assaulting a foreign envoy, but cleared of a charge under the National Security Law of assisting North Korea.
The Seoul Central District Court said he had “shown no repentance, attempting to justify his actions throughout the trial,” the AFP news agency reported.
Mark Lippert spent five days in hospital but has since returned to work and said the US mission in Seoul would remain “open and friendly”.
The US has some 28,000 military personnel based in South Korea, and the two militaries regularly carry out military exercises together.
The drills are an ongoing source of tensions with North Korea, which views them as preparation for war. They are also unpopular with many in South Korea, with demonstrations regularly staged against them.
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