Mayor Bloomberg’s appointment of one of his top government aides to his new private philanthropic foundation will help him lay the groundwork for giving away a large chunk of his personal fortune.

Mr. Bloomberg said yesterday that his commissioner for Human Resources Administration, Verna Eggleston, would quit her job to join the Bloomberg Family Foundation. She will be researching projects for the foundation to invest in.

The move comes a few months after Mr. Bloomberg purchased a $45 million building on the Upper East Side to house the foundation. Taken together, the two create the start of what could be Mr. Bloomberg’s post-mayoral career: Giving away his money.

It is unclear how his foundation will square with the mayor’s possible plans to run for president in 2008. He has repeatedly said he does not plan to run, but his actions, including his extensive national travel schedule, have suggested otherwise.

Mr. Bloomberg, whose net worth may be more than $20 billion, has given away hundreds of millions of dollars in the past and consistently ranks as one of the most generous philanthropists in the nation. For the most part, his donations have been done on a case-by-case basis, or through other foundations. In August, for example, he gave $125 million to five different organizations to fight smoking around the globe. This fall, the mayor quietly created the Bloomberg Family Foundation.

His tapping of Ms. Eggleston for a key role says a lot about how he views her accomplishments as HRA, the agency that oversees welfare. If he employs the same model for the foundation that he has had in government, he will be closely evaluating whether the programs invested in yield results.

As commissioner, she has overseen more than 15,000 employees at the nation’s largest municipal social service agency and reduced the welfare rolls to the lowest levels in 42 years.

Ms. Eggleston, a gay African American, has worked with the mayor since his first campaign for office and has been an important face for both his campaigns and his administration.

She has also established close relationships with the first deputy mayor, Patti Harris, and with Deputy Mayor Kevin Sheekey, each of whom worked with Mr. Bloomberg at his company before moving into government with him.

Mr. Bloomberg praised Ms. Eggleston yesterday. “Verna has done a great job,” Mr. Bloomberg said. “She wanted a change, and I thought it was a unique opportunity for my foundation. We’re going to focus on a number of social issues as well as public health issues and she will help us identify where the foundation can invest money.”

A spokesman for Mr. Bloomberg, Stuart Loeser, said possible replacements for Ms. Eggleston at HRA were being interviewed, but that no decisions had been made. He said Ms. Eggleston, who is the longest-serving HRA commissioner in the city’s history, would stay on through the transition period.