Starting his career with home town club Vitória de Guimarães, Mendes won the UEFA Champions League with Porto in the 2003–04 season.

Mendes was capped twelve times by the Portugal national team and was a member of their squad for the 2010 FIFA World Cup.

At Porto, he won the 2003–04 UEFA Champions League, Primeira Liga and the Cândido de Oliveira Super Cup under manager José Mourinho.

On 8 July 2004, Mendes joined Premier League side Tottenham Hotspur in a part-exchange deal which saw Hélder Postiga moving in the opposite direction.

He made only six appearances in the first half of the season, so it came as no surprise that in the January 2006 transfer window, then-Tottenham coach Martin Jol sought to offload him.

Mendes joined Portsmouth on 12 January 2006 in a combined deal that included Sean Davis and Noé Pamarot for a fee of £7.5 million.

Both goals were 25-yard shots, the first to give Portsmouth the lead on the hour mark and the second, in the 93rd minute, gave the club a 2–1 win.

[21] Mendes returned to Wembley Stadium three months later as part of the Portsmouth team that contested the 2008 FA Community Shield on 10 August.

[29] He started off the next season by playing against former club Porto in the Cândido de Oliveira Super Cup, which Vitória lost 2–1.

[31] After a six-year absence from international football, Mendes earned a surprise recall to the Portugal squad on 27 August 2008.

[32] He was given another surprise call up in October 2009, and started in the FIFA World Cup qualifier against Hungary, where he played the full match and set-up the second goal in Portugal's 3–0 win.

He was then included in Carlos Queiroz's 23-man squad to represent Portugal at the 2010 World Cup, where he took part in every match, two as a starter against the Ivory Coast and North Korea and two as a substitute against Brazil and Spain.