Tran, 38, faces up to 25 years in federal prison when he is sentenced in April.
In May, the former mayor was charged with bribery, extortion, attempted witness tampering and making false statements to a government agency.
Last year, Tran withdrew a guilty plea to a charge of accepting bribes while he was on the City Council.
Prosecutors allege that Tran accepted cash payments totaling $38,000 between 2005 and 2007 from a property developer. In exchange for the money, Tran allegedly promised to help obtain permits for a development project, according to court documents.
After the cash traded hands, Tran was voted out of office, the city never approved the project, and Tran never returned any of the cash, according to the U.S. Attorney's Office.
In a plea agreement, Tran admitted to telling the developer to give false testimony to a federal grand jury and to lying to the FBI when he told investigators that he never accepted money from the developer other than campaign contributions not exceeding $2,000.
Tran was elected to the Rosemead City Council in 2005 and held the post of mayor from 2007 to 2009. He is also a former El Monte Union High School District board member and onetime Assembly candidate.
City News Service contributed to this report.