Suhaib Majeed, 21, was convicted of conspiracy to murder and preparation of terrorist acts following a trial in the British capital.
His school friend, Tarik Hassane, 22, pleaded guilty to the same charges last month.
Jurors had been warned by the trial judge at the Old Bailey court to put Tuesday's deadly events in Brussels out of their mind as they considered their verdict.
Majeed, a physics student at London's King's College, had obtained a gun and ammunition and was discussing buying a moped when police arrested him, the court heard.
Hassane, seen as the ringleader, had been studying medicine in Sudan and was nicknamed "The Surgeon" but came back to Britain as the attack was planned.
Commander Dean Haydon, head of the Metropolitan Police's counter-terrorism command, told reporters that officers suspected Hassane "was communicating with people in Syria".
"There is every possibility that he was in Syria but I can't confirm that," he added.
The two men were said to have links to the west London mosque attended by Mohammed Emwazi, the IS figure known as Jihadi John who appeared in a string of execution videos before being killed in a drone strike in Syria last year.
"This is about acquiring a moped, committing a drive-by shooting, acquiring a firearm, a silencer and ammunition, and in almost in broad daylight targeting police officers and the military and members of the public and making good their escape," he said.
British police have foiled seven jihadist plots in the last 18 months, according to Home Secretary Theresa May.
No date has yet been fixed for the two men's sentencing.