Jon S. Corzine defended his tenure as chief executive of MF Global Holdings Ltd. before a House committee, but couldn't explain an estimated $1.2 billion in missing customers' money.

Visibly tense in the politically charged hearing, Mr. Corzine, 64 years old, on Thursday told the House Agriculture Committee that he has been "devastated by the enormous impact on many peoples' lives" caused by MF Global's collapse.

"I...