Gadahn, 28, who has become the first US citizen to be charged with treason in over 50 years, was born in 1978 and raised as Adam Pearlman.
His father Phil Pearlman, a 1960s rock musician and son of a prominent Jewish doctor, changed his name to Gadahn and converted to Christianity after choosing to raise his family in rural isolation on a goat farm.
Gadahn was home-schooled until he was 15, when he moved in with his grandparents in Santa Ana, California.
Relatives described Gadahn as a normal teenager, prone to fads, who grew his hair long for a while and developed a fondness for 'death metal' rock music.
"Adam was a very loving, caring, intelligent young man," said Gadahn's aunt, Nancy Pearlman, in a 2004 interview. "He was listening to hard rock music. He gave that up when he got religious."
At 17, Gadahn posted a statement on a University of Southern California website declaring he had converted to Islam, rejecting what he described as "demonic" heavy metal music.
"I discovered the beliefs and practices of this religion fit my personal theology and intellect as well as basic human logic," he wrote.
Gadahn's conversion came after he began attending the Islamic Center of Orange County, where he is believed to have come under the influence of two foreign-born Islamic radicals, Khalil Deek and Hisham Diab.
The Los Angeles Times cited family members of Deek and Diab as saying they were disciples of Sheikh Omar Abdel Rahman, who is now serving a life sentence for his role in the 1993 World Trade Center bombing.
Gadahn was reported to have been expelled from the Orange County mosque after attacking an employee and accusing staff of not being radical enough.
In 1998, Gadahn is believed to have left California for Karachi, before gradually falling out of contact with relatives in the US.
His parents are reported to have last heard from him in the months after the September 11, 2001 attacks, when he told them he had married an Afghan refugee.
In May 2004, US Attorney General John Ashcroft and FBI Director Robert Mueller named Gadahn as one of seven al-Qaeda members who were planning terrorist actions for later that year. No attacks ever materialised.
In October that year, US television broadcast a 75-minute videotape of a man who identified himself as "Azzam the American" threatening terrorist attacks in the United States.
"Allah willing, the streets of America will run red with blood, matching drop for drop the blood of America's victims," 'Azzam' says on the video.
Around the fourth anniversary of 9/11 in 2005, a man believed to be Gadahn appeared in a video warning of more attacks in Los Angeles and Australia.
"Yesterday, London and Madrid. Tomorrow, Los Angeles and Melbourne, God willing," he said, referring to 9/11 as the "blessed raids on New York and Washington".
But this year came the most striking evidence of Gadahn's growing role in al-Qaeda, when he appeared on propaganda videos also featuring the terror network's second in command, Ayman Al-Zawahiri.
In the most recent video, Gadahn appeared with his face uncovered and was introduced again as "Azzam the American", urging American forces to convert to Islam.