Randolph was pulled by the Portland Trail Blazers in the 19th position in the NBA Draft in 2001 and was first substitute of Rasheed Wallace. The rookie experienced an emotionally problematic season as the Portland players were known at that time, due to many problems with the law as the " Jail Blazers ". In the first two years, Randolph played 118 games, was good for an average of 8 points and 4 rebounds and played in 8 playoff games.
His breakthrough came in the season 2003/04. The Portland management separated by many weak-minded or dissatisfied players, and as Wallace went, Randolph became the new -starting power forward. He then scored 20 points and 10 rebounds per game, won the NBA Most Improved Player Award for the meistverbessertsten players and was rewarded with a lucrative contract of 84 million U.S. dollars for 6 years.
After Randolph injury missed the 40 games next year, he played an impressive season in 2006/7, where he scored 23 points and 10 rebounds per game. Therefore, he was brought in 2007 by New York Knicks coach Isiah Thomas to New York, and should help the Knicks team to new heights together with center Eddy Curry and Stephon Marbury playmaker. This plan failed 2007/8 miserably, as the Knicks won just 23 of 82 games this season and Randolph scored disappointing for him 17 points per game.
He moved with Mardy Collins in exchange for Cuttino Mobley and Tim Thomas on 21 November 2008 to the Los Angeles Clippers. There he played until the end of the 2008/2009 season, scoring an average of 20 points per game. In July 2009 it was announced that the Clippers Zach Randolph give in exchange for Quentin Richardson to the Memphis Grizzlies.
In 2010 he was nominated for the NBA All -Star Game and chosen team in the All- NBA Third 2011.