Mohammed Hamid, 50, of east London, was found guilty of training men in secret camps in the Lake District and New Forest to prepare them to fight abroad.
Among those to have passed through Hamid's camps were the four failed suicide bombers of 21 July 2005.
All four of the men responsible for the failed bombings were friends of Hamid.
The conviction marks a major success for counter-terrorism policing with Hamid regarded as a key figure in extremist networks.
Police say Hamid played a crucial role in grooming young men for terrorism and possible training overseas.
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Hamid's training came in the form of camping trips around the UK and late night talks in the living room of his home.
Prosecutors said he sought to groom impressionable young men, a process that had only intensified after the July 2005 suicide bombings.
Hamid, 50, was found guilty of three counts of soliciting to murder and three counts of providing terrorism training. The jury at Woolwich Crown Court found him not guilty of providing weapons training at camps.
The jury cleared him of two further counts of soliciting to murder, relating to speeches, and a further charge of possessing terrorist articles.
Seven other men have been convicted in connection with his camps in a series of linked trials that have been under a partial reporting black-out.
Hamid and Ahmet will be sentenced at a later date.
Another man, Mousa Brown, was cleared of providing or receiving training.
During the trial, the jury heard that Hamid had close links to the leader of the 21/7 bombings, Muktar Ibrahim.
The lead bomber and four of his conspirators had joined one of Hamid's camps in the Lake District in the spring of 2004, the trial heard.
He and Hamid had also worked the same preaching stall in central London in 2004, leading to their arrest after a row with a police officer. Hamid told the arresting officer he was "Osama bin London" and armed with a bomb.
On the evening after the 7/7 suicide attacks Hamid had texted one of the men who would two weeks later try to blow himself up.
The trial heard that Hamid had been preaching for up to 12 years around London, including at Speakers' Corner in Hyde Park, his east London home and in his Islamic bookshop.
An undercover police officer codenamed Dawood later infiltrated Hamid's gatherings and recorded him and Ahmet encouraging young Muslims to fight.
Hamid had denied radicalising young men and claims he was running a "drop-in club" for al-Qaeda. He said that while hundreds of young men had come into contact with him over the years, he was trying to offer them support because Muslims were vulnerable in the wake of 9/11.
Acting Assistant Commissioner Peter Clarke of the Metropolitan Police praised the jury for seeing through Hamid's jokey personality.
"If you want to judge how serious Hamid, Ahmet and the others were, you just have to look at the group," he said.
"The group included the people responsible for the attacks in London on 21 July 2005. You have to look at the fact that they celebrated the deaths that occurred on 7 July.
"And in fact what they were doing accelerated and intensified to try to mount attacks both here and overseas.
"You can only possibly conclude that these were serious determined terrorists."
