Kay worked as the UN Chief Weapons Inspector from 1991 to 1992. Following that, he was Vice President of Science Applications International Corporation (SAIC) from 1993 to 2002. While at SAIC, he worked alongside Steven Hatfill until March 2002. Then, he was appointed as a Special Advisor for Strategy regarding Iraqi Weapons of Mass Destruction (WMD) Programs. He received the International Atomic Energy Agency's Distinguished Service Award and the U.S. Secretary of State's Commendation. (SAIC was contracted by the U.S. to build prototype Mobile Weapons Laboratories in fall of 2001)
Following the U.S. invasion of Iraq, he returned to the country, working with the Central Intelligence Agency and U.S. military in 2003 and 2004 to determine if Saddam Hussein's regime had continued developing banned weapons. (See Iraq Survey Group)
The research of his team determined that the Iraqi unconventional weapons programs had mostly been held in check, with only small amounts of banned material uncovered (this included a number of vials containing biological agents stored in the home refrigerators of Iraqi scientists, for example). However, none of these substances had been “weaponized”—no such agents were found in missiles or artillery, and none could be easily installed. In an interview with National Public Radio, however, Kay revealed that Iraq had been working on weaponizing ricin up until Operation Iraqi Freedom. These discoveries indicate that some of the primary reasons President George W. Bush used for going to war with Iraq did not reflect the true situation in that country, and contradicted statements made by Kay himself in the lead-up to the war.
Before the 2003 war, as U.S. government officials were pushing the idea that Saddam Hussein was in possession of WMD, many people would direct reporters toward David Kay to reinforce their point of view. In September 2002, Kay told U.S. News & World Report that “Iraq stands in clear violation of international orders to rid itself of these weapons.” His credibility as a former U.N. weapons inspector convinced many observers.
On January 23, 2004, David Kay resigned stating that Iraq did not have WMD and that "I think there were stockpiles at the end of the first Gulf War and a combination of U.N. inspectors and unilateral Iraqi action got rid of them."  Kay was replaced in his role by Charles Duelfer and spent the following days discussing his discoveries and opinions with the news media and the United States political establishment. He testified on January 28, 2004 that “[i]t turns out that we were all wrong” and “I believe that the effort that has been directed to this point has been sufficiently intense that it is highly unlikely that there were large stockpiles of deployed, militarized chemical weapons there.” However, Kay defended the Bush administration, saying that even if Iraq did not have weapons stockpiles, this did not mean the nation wasn't dangerous. Kay also blamed faulty intelligence gathering for the prewar WMD conclusions. On February 2, 2004, Kay met with George W. Bush at the White House and maintained that Bush was right to go to war in Iraq and characterized Saddam Hussein's government as “far more dangerous than even we anticipated” when it was thought he had WMDs ready to deploy.
- Donald Barlett and James Steele, “Washington’s $8 Billion Shadow”, Vanity Fair, March 2007
- Borger, Julian (March 3, 2004). Admit WMD mistake, survey chief tells Bush. The Guardian (Guardian Unlimited).
- Lakely, James G. Bush confers with Kay, sets appointment of panel. The Washington Times.
- Whitelaw, Kevin (February 9, 2004). 'We Were All Wrong.' U.S. News & World Report, pp 24–25.
- DCI Tenet Announces Appointment of David Kay as Special Advisor (June 11, 2003). ‘Appointment’ 'Central Intelligence Agency'