Thomas F. O’Neill, a founding principal of Sandler O’Neill + Partners L.P., on Wednesday unveiled plans to retire from the New York banking firm.

“I have decided to retire from Sandler O’Neill and join Ranieri Aassociates as chairman of Ranieri Partners Financial Services Group,” he said in a memo sent out to the firm’s employees at midday Wednesday.

O’Neill began his Wall Street career at L.F. Rothschild & Co. in 1972. At Rothschild, he  specialized in financial institutions in the firm's bank service group. Twelve years later, he was appointed managing director of the group which then had 55 dealmakers.

O’Neill joined Herman Sandler in Bear Stearns’ financial services group in 1985, and became a Bear Stearns managing director and co-manager of the group.

The two opened up their own firm in 1988; at his specialty banking firm O’Neill has advised bank and thrift clients on strategic transactions and financings.

Last month, Sandler O’Neill announced that Kelso & Co. and The Carlyle Group will invest in the investment banking firm.

Specifics of the transaction were not disclosed, but Sandler O’Neill said in a press release that the minority equity investment will be “substantial.” The deal is to close by year-end. Proceeds from the investment, Sandler O’Neill said at the time, will be used to fund expansion plans.