The former midfielder revealed the lure of competing for titles and Champions League football in Scotland proved too appealing to turn down.
Yet, he described an amicable split after two-and-a-half memorable years at Fratton Park, to which his relationship with the club and its fans remained unscathed.
The combative and classy central-midfielder made a rare appearance on the south coast last Friday for the Lee Rigby Memorial Cup at AFC Portchester.
It was a return to the area where he amassed 68 appearances in royal blue – and played influential roles in the Blues’ Great Escape, their highest Premier League finish and the FA Cup triumph.
However, his outing in the Community Shield final three-months-later represented his final appearance for Harry Redknapp’s side, as he moved north of the border shortly after.
And the now 43-year-old explained how pushing for European football’s elite competition slightly outweighed playing in the UEFA Cup for Pompey in 2008-09.
Mendes told The News: ‘When I moved to Rangers, I knew they were in a difficult situation and wanted to move back to winning titles again.
‘So to be fighting for championships and Champions League places while hopefully playing in the Champions League again was important to the choice I made.
‘Rangers is also a massive club.’
Mendes’ move to Scotland signalled the end of his time in England, as he returned to his native Portugal after 18-months at Ibrox, before retiring in 2012.
During that time, he made 50 appearances for Walter Smith’s side and won the SPL title in his maiden season, before making two appearances in the Champions League in 2009-10.
And after achieving his Gers ambitions, he explained it was the right decision to end his Fratton chapter .
Mendes added: ‘At the time everything was so quick.
‘I think everyone was happy with what was happening and I think the deal was good for everyone, so I left in a good way with the club and the fans.
‘My relationship stayed great with everyone in the club and it was the right time to take another step and challenge in my career, that was it.’