The American billionaire’s Tornante investment group today sealed a £5.67m deal having been engaged in due diligence since May.
It ends the era of community ownership, which began in April 2013 and brought with it stability, promotion and the League Two title.
Eisner and his three sons, Breck, Eric and Anders, will now sit on the board, along with Tornante president Andy Redman and Pompey chief executive Mark Catlin.
And the new owners will be present at Fratton Park on Saturday for the visit of Rochdale in the League One curtain-raiser.
In a statement, Eisner said: ‘My family and I are thrilled to take on this incredible responsibility as stewards of a football club with such a rich heritage.
‘We have come to know the Pompey community in recent months and we are extremely impressed by their passion, dedication and love for their club.’
In addition, Catlin has been handed a long-term contract to remain as the Blues’ chief executive.
That retains his involvement in the day-to-day running of the club, while other members continue to be based in America.
Meanwhile, in the statement, the Eisner family also paid tribute to the Pompey Supporters’ Trust, the presidents and the Fratton faithful in recognition of their feats in saving and then stabilising the club.
Eric Eisner added: ‘The football club wouldn’t be here today if it wasn’t for the against-the-odds commitment of Pompey supporters who came together and saved this club. This is and will always be your team.’
The takeover ends supporter ownership in the club, although there will be a Heritage board created to allow fans’ continued input.
And Trust chairman Ashley Brown delivered his best wishes to Tornante.
He said: ‘The supporters fought hard to save Pompey, they’ve given the club incredible support under fan ownership, ensuring its successful promotion from League Two as champions.
‘Our passionate supporters now willingly pass the custodianship of Pompey to Michael Eisner, his family and Tornante.
‘It is clear that Michael and his family have caught the Pompey bug and we hope to see them regularly at Fratton Park, sharing many more successes on the pitch as part of the Pompey family.
‘The four Trust board members who have represented the Trust on the club board feel incredibly honoured and proud to have held that position at a club they love.
‘The many other fans who were involved in ensuring Pompey’s survival should feel equally proud of the part they played, and we join together to welcome Michael and his family to our city, our club and look forward to exciting times ahead.’