The 17-year-old was found guilty of assault, possession of an offensive weapon and one count of threatening a person with a bladed article at Stratford Magistrates Court on Friday, June 9.
He has been bailed for sentencing on a date to be confirmed in July.
He was caught after an incident unfolded when PC Nick Fox spotted two male youths on bikes acting suspiciously and appearing to look for phones to snatch in Kingsland Road at around 9am on Sunday, April 23.
PC Fox yelled at them and in response one of the males took out a knife and waved it at him.
He later spotted the youths as they were about to target a lone female victim. The officer shouted to alert her before one of the males approached him and drew a knife from his bag.
But the youth quickly jumped back on his bike and fled the scene after he noticed a Robbery Taskforce officer arrive.
A member of the public then pushed the youth off his bike, allowing PC Fox to move in and detain him.
He was arrested and taken into custody. A knife and three stolen mobile phones were found.
Detective James Conway said: “This is a fine example of bravery and excellent teamwork; an off-duty officer going above and beyond the call of duty, swiftly and ably assisted by his on-duty colleagues who rapidly responded to help the safe detention of the suspect.
“I am immensely proud of their efforts to help keep members of the public safe, but I’m not surprised. My officers and staff are out there every day, working tirelessly for Londoners. Then they go home, get up and do it all over again. This shows that even outside work they are putting the safety of the public first.
“I’d also like to thank the woman who helped stop the male, but left the scene before we could speak to her. Her action showed real courage and a selfless determination to assist the police preventing harm.”
