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 Czechoslovakvoucher privatization, he founded the investment fund PPF 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 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 arm 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. From 2007 till 2013 Kellner was member of the Board of Directors of Generali. Kellner himself later bought 2.02% stake in Assicurazioni Generali. In 2013 Kellner's PPF sold their remaining shares in the Generali PPF Holding to Generali for €2.5 billion, who became sole owner effective at year end of 2014.
Wealth
As of 2013, Kellner owned 98.93% of the PPF investment and financial group. In 2006, Forbes estimated PPF's net worth at US$3 billion and listed Kellner as 224th wealthiest man in the world. In 2007, he moved up to 119th with US$6 billion and then in 2008 to 91st with US$9.3 billion. In 2009 he was 76th with US$6 billion and in 2010 89th with US$7.6 billion. Kellner's 2015 position on the Forbes list was 137th with an estimated wealth of US$9.2 billion. He was estimated to have a net worth of $11.4 billion according to Forbeslist of billionaires, making him the wealthiest person in the Czech Republic and the 102nd richest person in the world as of 10 August 2016. In March 2016, his net worth was $10.9 billion. However, the above figures were criticized as overestimated by some Czech economic media. Their estimates were considerably lower, CZK 60–75 billion according to the weekly Týden, or even less than CZK 50 billion according to journalist Miroslav Motejlek. In 2018, Petr Kellner ranked #88 on the Forbes World's Billionaires list, with wealth listed at US$15.5 billion. 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.
