Petr Kellner graduated from the University of Economics, Prague Faculty of Industrial Economics, in 1986. After the Velvet Revolution he worked for the Czech company Impromat, an importer and seller of Ricoh photocopiers. While working for that company, he met Milan Maděryč and Milan Vinkler.

In 1991, after the announcement of Czechoslovak voucher privatization, he founded the investment fund PPF (První privatizační fond) with Milan Vinkler and state-owned glassworks Sklo Union. In 1992, PPF investiční společnost a.s. was established and the funds’ names were changed from privatisation to investment. The funds were very successful and purchased stock of more than 200 corporations with nominal value 5 billion CZK.

In 1995 and 1996, PPF bought a 20% stake in the largest Czech insurance company, Česká pojišťovna and started to manage it. In 2000, PPF bought 31.5% stake of Česká pojišťovna from IPB for 2.85 billion CZK . The transaction with IPB ended up with IPB paying to PPF 1.6 billion CZK penalty, making the real purchasing cost of the stake 1.25 billion CZK. Later PPF acquired more shares and became the dominant (93%) owner in 2001, when PPF bought large stakes of shares from Komerční banka and the Czech state. With help of these profitable transactions, the last Investors from voucher privatization were bought out and Petr Kellner became the dominant owner of PPF group.

In 2007, PPF group signed a contract with Assicurazioni Generali to create a joint venture between PPF Group’s insurance branch and Assicurazioni Generali's corporations in the Czech Republic, Slovakia, Poland, Hungary, Romania, Bulgaria, Ukraine, Russia, Serbia, Slovenia, Croatia, Belarus, and Kazakhstan. According to sources PPF Group will own 49% of that company from the start, and Generali will pay €1.1 billion to the PPF Group for its stake. Petr Kellner became member of the Board of Directors of Generali. Kellner himself later bought 2.02% stake in Assicurazioni Generali.

Formerly, he owned a Gulfstream G500 airplane named Káně. In 2010 he bought a Boeing 737-700BBJ as his Business airplane. He lives in an extensive residence in the village of Podkozí near Prague, but he owns a number of other real estates in various countries. He also owns one of the largest collections of photographs taken by Josef Sudek.

However, the above figures were criticized as overestimated by some Czech economic media. Their estimates were considerably lower, CZK 60–75 billion (approx. US$4 billion) according to the weekly Týden, or even less than CZK 50 billion (app. US$3 billion) according to Journalist Miroslav Motejlek. Thus, Kellner would have been surpassed by Entrepreneur Andrej Babiš on the position of the wealthiest resident of the Czech Republic.