When Mr. Thomson was forced out in 2007, Mr. Weill was one of his first calls: “I unloaded to Sandy,” Mr. Thomson says.

For Mr. Weill, calls like these  coupled with the collapsing share price  burned; they made him want to act.

Starting in late 2007, he began approaching some members of Citi’s board about returning to help with its recovery. He tried first when the board was looking to replace Mr. Prince as C.E.O., and later after Vikram Pandit got the job. At the time, Mr. Weill imagined that he would be welcomed. “I had 50 years of experience,” he says. “I think I was a pretty good student of the markets, and the business. I had a good feel of things. I felt that just because I retired didn’t mean my brain went to mush. Maybe I could help.”

No one responded to his offers.

The rejection stung. Citigroup had for so long been central to his life. It was hard to accept that he had no control or influence over it anymore. “It’s very hurtful. Even though he says, ‘No, no, it’s fine,’ ”says Joan Weill, his wife of 54 years. “I know him. The company means so much to him. It was his baby.”

Mr. Weill continued to track it closely. “He was watching every movement of the stock; he was reading everything,” recalls Mike Masin, a longtime friend and a former chief operating officer of Citigroup. “We have had conversations about the fact that he has to make Citi less a part of his life.”

One news item, in particular, was crushing: Last winter, The New York Post ran a picture of Mr. Weill on its front page with the headline, “Pigs Fly: Citi Jets Ex-C.E.O. to Cabo.” He had taken the corporate plane to vacation in Mexico, weeks after Citi had accepted a $45 billion taxpayer bailout. The flight provoked a public outcry and media frenzy.

Mr. Weill says he was horrified by being cast as a greedy, out-of-touch Wall Streeter taking advantage of taxpayers. That is not how he sees himself or how he wants the public to see him. The night the Post article came out, he issued a press release promising to never again use the Citi jet.