Alternately cursing his public defenders and the United States, the self-proclaimed "slave of Allah" also gave a very self-controlled testimony, even if it was self-incriminating.
His character will prove crucial to those told to decide if the only man charged in connection with the 2001 attacks in the United States, will be executed.
Moussaoui's journey to notoriety began with a deprived childhood in France, took him to a radical London mosque, to training camps in Afghanistan and a failed attempt to learn to fly a jet.
Early beginnings
Born in 1968 in Morocco, he grew up in France, with the first five years of his life, according to his lawyers, were spent in and out of orphanages.
His mother, Aicha el-Wafi, married at 14 and beaten by her alcoholic husband, eventually opted for divorce.
Moussaoui, his older brother, Abd Samad, and his two sisters, Nadia and Jamily, heavily relied on one another throughout their childhoods.
At the age of 22 Moussaoui, who speaks English, French and Arabic, moved to London to study international business at South Bank University.
It was at this time, according to his brother, that he started to adopt aggressive and intolerant rhetoric, critical of the West and started going to radical mosques in the British capital including one in Finsbury Park where Abu Hamza was one of the preachers.
Moussaoui’s mother, Aicha el-Wafi told AFP that she believed terrorist recruiters exploited her son's revulsion at "plain racism," yearning for a lost father, and deep emotional emptiness -- to stir his radicalism.
Al Qaeda links deepen
Moussaoui visited Chechnya, where Muslim rebels are fighting for their own state, and in 1998 went to an Al-Qaeda training camp in Afghanistan.
From 1999 onwards, French authorities started to keep a record on Moussaoui.
On a visit to Malaysia in 2000 Moussaoui stayed with Al-Qaeda leaders and the future September 11 hijackers.
According to the independent US commission which investigated the September the 11th attacks, Moussaoui met Khaled Sheikh Mohammed, the alleged mastermind behind the attacks.
Sheikh Mohammed and other radicals, now in US custody, said in testimony read in court that they believed Moussaoui was "not right in the head" and so he was taken off the plans for the devastating strike on the United States.
Next he traveled to the United States, taking flying lessons in Oklahoma before moving again to Minnesota where he trained on a 747 Flight Simulator.
His erratic behaviour drew the attention of his instructors who tipped off the FBI.
Moussaoui was arrested in the United States for overstaying his visa.
Last year, Moussaoui signed a statement of fact along with his guilty plea identifying himself as "the 20th hijacker". But he also maintained that he was to form part of a second wave of strikes, probably targeting the White House.
He has rejected the public defence lawyers named by the court and regularly condemned them in hearings.