"Their plight weighs on my mind every day - every hour," Corzine, a former US senator, said in 21 pages of remarks prepared for delivery before the House Agriculture Committee.
"I simply do not know where the money is, or why the accounts have not been reconciled to date," he said.
Corzine also distanced himself from some hands-on aspects of the firm's business practices.
"Even when I was at MF Global, my involvement in the firm's clearing, settlement and payment mechanisms and accounting was limited," he said. "I was stunned when I was told on Sunday, October 30, 2011, that MF Global could not account for many hundreds of millions of dollars of client money."
Corzine's contrite but defensive remarks are his first since MF Global's October 31 bankruptcy and his resignation days later. Revelations of massive bets on European sovereign debt caused markets to lose confidence in the firm.
The search for hundreds of millions of dollars in missing customer funds has sent reverberations through the farm belt and trading floors, and has attracted the attention of the FBI and federal prosecutors. Thousands of customers have had their money frozen.
"It appears to me that nobody has learned a thing from what's gone on here. Wall Street is operating as if 2008 never happened," said Collin Peterson, the top Democrat on the Committee, referring to the recent financial crisis.
In separate testimony, a top executive of futures exchange operator CME Group said MF Global misused hundreds of millions of dollars of customer funds by moving the money to its own accounts, the strongest accusation yet against the bankrupt futures brokerage.
"Transfers of customer funds for the benefit of the firm constitute serious violations of our rules and of the Commodity Exchange Act," CME Executive Chairman Terrence Duffy said in prepared remarks.
CME, the biggest US futures exchange operator, was a hands-on regulator of MF Global. Duffy said the brokerage admitted during a call with regulators that customer money was transferred out of segregation to the firm's own accounts.
Neither MF Global nor any of its executives has been charged with wrongdoing.
Nine witnesses are scheduled at the hearing, but Corzine, a senator from 2001-2006 and a former governor of New Jersey, is the star.
Corzine, due to testify later in the day, arrived just before the hearing started with little fanfare. Capitol Hill Police quickly escorted Corzine to the committee's holding room where he was expected to monitor the first panel.