The 39-year-old Welsh mezzo-soprano was attacked after intervening in a street robbery as she went to rehearse for a carol concert, on 4 December.
At Highbury Corner Youth Court, the girl admitted stealing Ms Jenkins’ iPhone and assaulting a police officer.
The teenager was handed a six month referral order. She has offered to apologise to Ms Jenkins.
The singer was on her way to a rehearsal for the Henry van Straubenzee charity event, when she witnessed an “older lady being mugged” and intervened to help, her agent said.
“Katherine was then mugged herself,” her agent added.
At the hearing, district judge Susan Williams also ordered the girl’s mother to pay £20 in compensation.
Sabrina Fitzgerald, the girl’s counsel, said the teenager took the phone “because she thought she was being filmed”.
There were “issues around peer pressure and poor decision-making skills”, she added.
Ms Jenkins was not in court for the hearing.