An eight-member House ethics committee has found Democratic Representative Charles Rangel of New York guilty on 11 counts of breaking House rules.
The panel will make a recommendation to the House of Representatives on appropriate punishment for Mr Rangel following a hearing.
The 80-year-old had been accused of 13 counts of engaging in financial and fundraising misconduct.
He is a former committee chairman with 40 years' service in Congress.
The panel reached its decision after a second day of closed-door discussions. But the congressman was not present when the verdict was announced.
The senior New York congressman walked out of proceedings on Monday in protest at the panel's refusal to postpone the hearing while he found a new lawyer.
Possible punishment options the committee may consider include a House vote deploring Mr Rangel's conduct, a fine and denial of privileges.
"We have tried to act with fairness led only by the facts and the law, and I believe that we have accomplished that mission,'' said Democratic Representative Zoe Lofgren of California, who chairs the ethics committee.
The panel found that Mr Rangel had used House of Representatives headed paper and congressional staff to solicit money for a New York college centre named in his honour.
The group also found Mr Rangel had failed to disclose at least $600,000 (£378,000) in assets and income in a series of inaccurate reports to Congress.
The congressman was also found guilty of improperly using a rent-controlled residential apartment as a campaign office and failure to report rental income to the Internal Revenue Service from letting out a house in the Dominican Republic.
But the panel could not reach a conclusion on one count concerning whether Mr Rangel had inappropriately accepted gifts. It also combined two charges surrounding the congressman's misuse of congressional stationery into one count.
Mr Rangel was first elected to Congress in 1970 from a heavily Democratic district in New York City's Harlem district.
Despite the charges against him, he won re-election on 2 November with 80% of the vote.
He stepped down as chairman of the House Ways and Means Committee, which handles tax legislation, amid the ethics allegations in March.