Chris Parker claimed to have helped some of those injured in the attack and was initially hailed a hero, with more than £50,000 being raised by the public to help the rough sleeper.
But CCTV recovered by police showed him wandering the foyer of the arena amid the dying and injured, stealing from victims.
They included injured survivor Pauline Healey, whose her 14-year-old granddaughter lay dying nearby.
Sorrell Leczkowski, a pupil at Allerton High School in Leeds, was one of 22 people who died in the blast on May 22 last year.
Parker, 33, admitted at a hearing at Manchester Crown Court last month to stealing a purse and a mobile phone from two victims.
He faces sentence at the same court and "most likely" faces a jail sentence, Judge David Hernandez warned him.
Parker was caught on camera leaning over the body of Mrs Healey to steal her purse.
He also snatched the mobile phone of a teenager and within hours was using Mrs Healey's bank cards at a local McDonald's.
Prosecutors said the defendant provided "some limited assistance" but that he also took the opportunity to commit the thefts immediately after the atrocity.
Parker will be sentenced for two counts of theft and one count of fraud.