Richardson was reprimanded for forcing her congressional staff to perform campaign work and violating government service codes of conduct. She was fined $10,000 as a result.
Richardson agreed to accept the reprimand. She had admitted to all seven counts against her.
Richardson criticized the House Ethics Committee for the way they conducted the investigation and denied that she had coerced her staff.
"I want to emphasize that I have never taken or threatened any action against any staffer who did not volunteer to work on my campaign," she said. "There is no doubt that a number of staff felt compelled or coerced to do so. That was not my intent, and I deeply regret that this occurred."
The Ethics Committee report, which was adopted unanimously, said that employees in her Long Beach office were compelled to do campaign work after their workdays and that those who declined were given the impression they could lose their jobs. It said Richardson sought to influence the testimony of staff members before the Ethics Committee by suggesting they tell the committee their work on her campaign had been voluntary.
It said that in the fall of 2010, she directed a staff member to volunteer for her opponent's campaign under a fake name to gather information. It said the coercion of staff began in early 2010 and continued in the current campaign, even though Richardson knew she was under investigation.
"The conclusion was bipartisan, and it was unanimous," said Rep. Linda Sanchez of California, top Democrat on the panel, which is made up of five Republicans and five Democrats.
Because of redistricting, Richardson is locked in a tough re-election race against Hahn, who defeated her by a 60-39 margin in the primary. California law allows the top two finishers to run against each other in the general election, regardless of party affiliation.
Richardson's 37th District includes communities in the city and county of Los Angeles, including Carson, Compton, Long Beach and Signal Hill.
The Associated Press contributed to this report.