He reached the semi-finals of the 1998 French Open, won the 2003 Rome Masters, and achieved a career-high singles ranking of World No.

Mantilla began playing tennis at the age of ten and was a member of the winning Spanish Sunshine Cup team along with Albert Costa in 1992.

On the ATP main tour, Mantilla qualified for his first TMS event in Monte-Carlo and defeated the two time Roland Garros champion Sergi Bruguera before losing in the quarter-finals to Albert Costa.

In the week before the French Open, Mantilla reached his second final in St Pölten losing to Marcelo Ríos.

After a disappointing second round exit to Magnus Larsson at Roland Garros, Mantilla went on a hot streak and won four titles in his next five tournaments at ATP level and in between made the final of the Braunschweig Challenger losing to Francisco Roig.

Mantilla's run began at Bologna where defeated reigning French Open champion Gustavo Kuerten, then followed up with titles in Gstaad over Joan Albert Viloca, in Umag over Sergi Bruguera and in San Marino over Magnus Gustafsson.

He beat Wolfgang Schranz, Byron Black, Fabrice Santoro and Ramón Delgado to reach the quarter-final.

Apart from his title he reached the semis of the TMS events in Monte Carlo and Rome losing to Gustavo Kuerten and to Pat Rafter respectively.

Mantilla experienced the first of his shoulder problems and underwent surgery in 2000 and was sidelined for six months and did not return to tennis until January 2001.

He won two consecutive Challengers in Espinho over Tommy Robredo and Barletta defeating Markus Hipfl, this was followed up with a final in Estoril losing to nemesis Juan Carlos Ferrero as he did in all 8 of their matches.

Mantilla made two finals in 2002 in Doha and Indianapolis on hardcourt losing to Younes El Aynaoui and Greg Rusedski respectively.

In May Mantilla won the Internazionali d'Italia at the Foro Italico in Rome, his last and biggest career title to date, defeating the favourite Roger Federer 7–5, 6–2, 7–6 and becoming the first unseeded player to win the event.

[7] Mantilla returned to tennis in the 2007 Monza Challenger and qualified for the main draw winning his 3 qualification matches.

He owns a large collection of Spanish movies and his favourite actors are Gabino Diego, Coque Malla and Juanjo Puigcorbé.