Horna turned professional in 1998 and he moved up over 1,000 places in the rankings with victories in the Ecuadorian, where he defeated Sergio Roitman as a qualifier and three Futures events in Peru and in 1999 made his first ATP Challenger final in Aschaffenburg.

In 2000 he was finalist in Salinas and again in Aschaffenburg and it was not until 2001 that Horna was able to get his first win on the ATP tour in Umag defeating Martin Damm.

Horna won the Bermuda Challenger over Martín Vassallo Argüello and made his first ATP final in Long Island losing to Lleyton Hewitt.

Despite Horna winning his first ever ATP singles title defeating Juan Ignacio Chela of Argentina 7–6(5), 6–4 in Acapulco.

[2] As well as reaching the third round of the Australian Open for the first time defeating Gaël Monfils before losing to Paul-Henri Mathieu and winning his second doubles title with Martín García in Palermo.

Horna had an unfortunate start to 2007 by losing his first round match at the Australian Open to doubles' specialist Max Mirnyi, after being frustrated by the umpire's refusal to eject an abusive heckler in the fifth set.

In February of that year he won his second ATP singles title, defeating Nicolás Massú for the only time in 7 matches 7–5, 6–3 in Viña del Mar, Chile, without losing a set in the tournament.

In September, Horna and Iván Miranda took the Peruvian team of Davis Cup to the World Group for the first time by beating Belarus in Lima 4–1.

While Horna has only made one semi final in 2008 in Acapulco, he has won 3 doubles titles in Auckland with Juan Mónaco, in Buenos Aires with Agustín Calleri and the 2008 French Open with the Uruguayan Pablo Cuevas.

Horna and Cuevas by virtue of winning Roland Garros had qualified for the Tennis Masters Cup doubles where they made the semi-finals losing to Nenad Zimonjić and Daniel Nestor, by finishing second in their round robin group behind Bob and Mike Bryan.

2009 was Horna's last season on tour, and played his final tournament at Lima Challenger,[7] where he lost in the second round to Chilean Jorge Aguilar.