Ponomariov was born in Gorlovka in Ukraine. In 1994 he came third in the World Under-12 Championship at the age of ten, and the following year won it at the age of eleven. In 1996 he won the European Under-18 Championship at the age of just twelve, and the following year won the World Under-18 Championship. In 1998, at the age of fourteen, he was awarded the Grandmaster title, making him the youngest ever player at that time to hold the title.
Among Ponomariov's notable later results are first at the Donetsk Zonal in 1998, 5/7 in the European Club Cup 2000 (including a victory over then-FIDE World Champion Alexander Khalifman), joint first with 7.5/9 at Torshavn 2000, 8.5/11 for Ukraine in the 2001 Chess Olympiad in Istanbul, winning gold medal on board 2, and first place with 7/10 in the 2001 Governor's Cup in Kramatorsk.
In 2002 he beat his fellow countryman Vasily Ivanchuk in the final of the FIDE Knock-Out World Championship by a score of 4.5-2.5 to become FIDE world champion at the age of eighteen, the first teenager to ever become world champion. In the same year he finished second in the very strong Linares tournament behind Garry Kasparov. His reult in the strong 2003 Corus tournament at Wijk aan Zee was less good - despite having the third highest Elo rating, he finished only join eleventh out of fourteen players with 6/13, and at Linares the same year he finished only fifth out of seven with 5.5/12 (although as this was a very strong tournament, this wasn't necessarily a bad result).
There were plans for him to play a fourteen game match against Kasparov in Yalta in September 2003, the winner of which would go on to play the winner of a match between Vladimir Kramnik and Peter Leko as part of the so-called "Prague Agreement" to reunify the world chess championship (since 1993 there have been two world chess championships). However, this was called off after Ponomariov refused to sign his contract without reservation. He will remain FIDE champion until the winner of the Knock-Out World Championship has been determined at the end of 2003.
On October 11, 2003, Ponomariov became the first high-profile player to default a game because of his mobile phone ringing during play. This happened in round one of the European Team Championship in Plovdiv, Bulgaria, when Ponomariov was playing black against Evgenij Agrest.
In his games with white, Ponomariov has almost always played 1. e4 (see algebraic notation and chess opening), entering the main lines of the Ruy Lopez and Sicilian Defence. With black, he has played the Sicilian against 1. e4 and also replied 1... e5, going into the Ruy Lopez. Against 1. d4 he has adopted a variety of defences, including the Queen's Gambit Accepted, the Queen's Indian Defence and the King's Indian Defence. Earlier in his career he experimented with the Benko Gambit and Pirc Defence, but as of 2003 these have fallen out of his repertoire.
In the January 2004 FIDE Elo rating list, Ponomariov had a rating of 2722, making him number nine in the world and Ukrainian number one.