The former Walt Disney CEO completed his purchase of the eve of the new season and attended the 2-0 home win over Rochdale.
Aged 75, Eisner is one of the oldest owners in English football. His Tornante investment group offered £5.67m to buy 100 per cent of the club and they will invest an extra £10m.
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 looked at MLS in the United States, around Europe and in the UK. We had seen a number of other opportunities and there was no other fan-base that came close."
Eisner, whose net worth is estimated at $1 billion has taken his place on the club's board, alongside his three sons, Breck, Eric and Anders, will make up the club's board along with Andy Redman, president of Tornante, and Portsmouth chief executive Mark Catlin.
"We bought Portsmouth because we recognised that the fans are fantastic and we recognised that Portsmouth and Fratton Park are unique.
"We also recognised that when we came here and met the people involved in this club, that we walked into a perfect situation."
Eisner has also pledged to form a heritage board which would protect the club's name, colours and badge, while preventing relocation to a ground more than 15 miles from Portsmouth city centre.