Lee said after the hearing that he would appeal the court's decision.
"I will be appealing. Beginning with the basic facts, it is a conclusion that is difficult to accept," he said on Friday.
If any prison sentence or fine of one million won ($A1100) or more becomes final, Lee would be stripped of his parliament seat and his eligibility to run in any elections for the next five years.
When Lee arrived at the Seoul Central District Court, his supporters chanted his name as he entered, while a group of protesters also gathered near the court.
Lee, head of the main opposition Democratic Party who narrowly lost to President Yoon Suk-Yyeol in the 2022 election, faces at least four trials after being indicted on several criminal charges including bribery and corruption.
The court ruled on Friday on charges he breached the election law by making false statements as a presidential candidate in 2021 that he was not acquainted with a city official who was in charge of a development project.
The project in the city of Seongnam where he had served as mayor has since become the centre of controversy bringing corruption allegations against Lee.
Lee was also accused of making a false claim during a parliamentary audit in 2021 about a land development project in Seongnam when he served as mayor.
Prosecutors had sought a two-year jail sentence.
Lee is widely expected to run in the 2027 presidential election.Â
In January, he survived a knife attack when he was stabbed in the neck by a man during an event and underwent surgery.
His party then went on to score a landslide victory in a parliamentary election in April, dealing a blow to President Yoon and his ruling party.
Lee, who faces another sentencing hearing over perjury charges later in November, and his party have accused prosecutors of pursuing a politically motivated case against him.
