Born in New York in 1953, Timmerman obtained a BA from Goddard College in 1973 and an M.A. from Brown University in 1976. He moved to France, where he pursued a career as a novelist, publishing a novel called Wren Hunt in 1976 and a novella called The Iskra Scrolls in 1980.

In the early 1980s, Timmerman became a Middle East correspondent for The Atlanta Journal-Constitution and developed an expertise in the Middle East and the arms trade. In 1982, he was taken prisoner for 24 days by Fatah guerillas in Lebanon. He was the first journalist on the scene when Islamic militants bombed the US Embassy in 1983.

From 1985 to 1987, Timmerman was a correspondent for Defense and Armament Newsweek and Military Technology, covering the Iran—Iraq War and the arms industry in the Middle East. He won the Joe Petrosino Prize for Investigative Reporting in 1987 for an investigation of an Iranian arms procurement group.

From 1987 to 1993, Timmerman published the Middle East Defense News and was international correspondent for Defense Electronics. He also wrote monographs for the Simon Wiesenthal Center on alleged efforts by Iraq, Syria and Libya to acquire weapons of mass destruction.

In 1991, Timmerman published The Death Lobby: How the West Armed Iraq after the Gulf War. Timmerman advised the United Nations Special Commission for the Disarmament of Iraq on the location of alleged secret weapons plants.

In 1993, Timmerman returned to the US where he worked as a member of the staff of the U.S. House Committee on International Relations. In 1995, he founded the Foundation for Democracy in Iran with Peter Rodman, Joshua Muravchick and Iranian opposition expatriates to attempt to topple the Iranian government. He founded the Middle East Data Project to advise governments and private companies. In 1998, he made suggestions to the Rumsfeld Commission supporting the deployment of a national missile defence system.

In 1998, he wrote a piece on Osama Bin Laden and his training camps in Afghanistan just before Al-Qaeda attacked two US embassies in Africa. He also wrote features for the American Spectator criticising the export of high-technology equipment to China, which was published as a book in 2000. In 2000 Timmerman sought the nomination of the Maryland GOP to run against popular Democratic incumbent Paul Sarbanes. Timmerman won less than ten percent of the party primary voting; Paul Rappaport won the Republican nomination but lost to Sarbanes, who won 63% of the vote.

Timmerman wrote Shakedown: Exposing the Real Jesse Jackson as a change of pace from his focus on international issues in 2002. The argument claimed that Jackson alleging connections with criminals and claiming that Rev. Jackson practised extortion of businesses. It proved to be highly successful making the top ten bestseller list with 200,000 copies printed. It also reached the top of the Amazon bestseller list.

In 2003, Timmerman published Preachers of Hate: Islam and the War Against America criticising Islamic preachers for hate speech. The French Betrayal of America was published in 2004. Timmerman returned to his field of greatest expertise with the publication of Countdown to Crisis: the Coming Nuclear Showdown with Iran in 2005.

On February 7, 2006 Sweden's former deputy prime minister and Liberal party leader Per Ahlmark asserted that he had nominated Timmerman for a Nobel Peace Prize along with UN Ambassador John Bolton for "their repeated warnings and documentation of Iran's secret nuclear buildup and revealing Iran's 'repeated lying' and false reports to the International Atomic Energy Agency." However, the Nobel Foundation doesn't confirm nominations until 50 years have passed.