Gadahn grew up on a goat farm in Riverside County, California, and converted to Islam at a mosque in nearby Orange County.
He moved to Pakistan in 1998, according to the US Federal Bureau of Investigation (FBI), and is said to have attended an al-Qaeda training camp six years later, serving as a translator and consultant for the group.
He has been wanted by the FBI since 2004, and there is a $1m reward for information leading to his arrest or conviction.
The 31-year-old is known by various aliases including Yahya Majadin Adams and Azzam al-Amriki.
He has posted videos and messages calling for the destruction of the West and for strikes against targets in the US.
His most recent video, posted on Sunday, saw him praising the US army major charged with killing 13 people in Fort Hood, Texas, as a role model for other Muslims.
Dawud Walid, the executive director of the Michigan chapter of the Council on American-Islamic Relations in Southfield, Michigan, condemned Gadahn's call for violence.
Walid, himself a navy veteran, said Muslims who have honourably served in the American military will be unimpressed by al-Qaeda's message aimed at their ranks.
"We thoroughly repudiate and condemn his statement and what we believe are his failed attempts to incite loyal American Muslims in the military," he said.