Hreidarsson, who made 123 appearances in five years with Pompey, is now manager of Icelandic club IBV and brought his new side to Fratton Park for a fundraising friendly on Tuesday night.
The match was also a celebration after the Pompey Supporters' Trust got the go-ahead to complete their rescue of the stricken club last week.
Pompey's relegation to League Two may have been confirmed on the same night after Oldham beat Yeovil, but 6,780 fans were in attendance to watch the likes of Hreidarsson and fellow old boys such as Svetoslav Todorov, Linvoy Primus and caretaker boss Guy Whittingham take the field.
And Hreidarsson said: "It was a big relief for everyone when the deal was agreed with the Pompey Supporters' Trust last week.
"It's absolutely massive. It's a new era for the club and everyone involved now has its best interests at heart.
"Now hopefully Pompey can start moving in the right direction and the right decisions will be made."
Portsmouth won the game 2-1 thanks to goals from Bradley Tarbuck and Liam Walker.