Sgt. 1st Class Randy Johnson, 34, was killed by an roadside explosive device that the defendant, Anis Sardar, helped make.
"Don't let me die here," Johnson begged his fellow soldiers as he lay wounded.
After his death, he was awarded the Purple Heart.
Sardar, 38, was convicted in London's Woolwich Crown Court. He is set to be sentenced Friday, and he faces the possibility of a life sentence.
Sardar was linked to the bomb plot seven years after the murder following a painstaking investigation involving analysis of the devices in an FBI lab. When anti-terrorist police raided his home in Wembley, in northwest London, in September, they found a bomb-making manual written in Arabic.
He originally denied any involvement in making the bombs and insisted he was studying Arabic in Damascus, Syria, when the devices were built.
But the IED that killed Johnson bore the fingerprint of Sardar's accomplice, Sajjad Adnan, whose whereabouts are not known. And eventually Sardar, whose fingerprints were found on tape pulled from two other bombs, admitted helping Adnan make explosive devices in Iraq.
The taxi driver, a University of Westminster dropout, told jurors he was in a "lawless war zone" and only wanted to defend fellow Sunni Muslims against attacks from Shia militia forces. He maintained that the real culprits were former British Prime Minister Tony Blair, former Vice President Dick Cheney and former President George W. Bush.
But the prosecution said Sardar was an expert bomb-maker who planned to slaughter coalition forces with a series of improvised explosive devices planted near Baghdad in 2007.
"Whoever made the bombs we are considering did so with murderous intent," prosecutor Max Hill told the jury of seven women and five men.
The jury deliberated a bit over 11 hours, then found Sardar guilty of murdering Johnson on September 27, 2007.
Sardar, who grew up in West London after arriving from his native India in his teens, said he went to Iraq in 2006 after studying Arabic in Syria. He told jurors he wanted to help prevent "religious ethnic cleansing" by Shia Muslims and said his involvement with the bomb-making was limited to helping wrap tape around the devices on a single day.
But Hill, the prosecutor, said the bombs were intended to kill and injure American soldiers operating out of Camp Liberty, a military base near Baghdad and the infamous Abu Ghraib prison.
Sardar was "a guilty participant in this deadly trade," Hill said, "making bombs so large that they could and did cause significant damage to heavily armored U.S. military vehicles, killing the unfortunate Sgt. Johnson."