The West Ham youth product has put pen to paper on a one-year deal, with the club having the option of a further 12 months.
He had been on trial at Fratton Park over the summer after leaving Charlton at the end of last season.
The triallists signs a week after The News reported he was prepared to take legal action against the club claiming breach of contract.
Blues boss Danny Cowley said he was glad to finally sign the youngster but revealed a loan move to aid his development is in his plans.
Cowley said: ‘Jay came in and really impressed us with his energy, power and athleticism.
‘Unfortunately he picked up an injury and we’ve been working hard with him on his rehab.
‘Now that he’s back fit, we’ve decided to sign him.
‘It gives us a good group of talented young professionals, along with Haji Mnoga, Liam Vincent and Jayden Reid.
‘The plan is for Jay to go out somewhere and play regular games.
‘This, accompanied with a good training programme, will hopefully see him develop as we hope.’
Mingi made two senior appearances for Charlton, featuring in Papa John’s Trophy games against Brighton U21s and Leyton Orient.