Attacking midfielder Leto was sold to Liverpool in the January transfer window in 2007, but played on at Argentinian Lanús until he officially joined Liverpool on 1 July. The young Argentinian appeared in four competitive matches in Liverpool's first team during the 2007/08 season. His debut came in the away leg of the Champions League qualifier in Toulouse. A month later he played the full 90 minutes of the League Cup win at Reading. He also featured from the start against Cardiff in the League Cup and Marseille in the Champions League but was substituted on both occasions, as indeed he had been at Toulouse. Liverpool were denied a work permit for him in the summer of 2008 and he agreed to a two-year loan at Greek side, Olympiacos. Halfway through his loan period there was interest from another Athens club in Panathinaikos. This led to the player making a permanent move away from Liverpool on 1 July 2009 with the fee believed to be around £3 million. That was good business for Liverpool as Leto had cost them less than £2 million two years earlier. Leto, who made no impact in England, became an instant hero in Greece when he scored the winning goal in the 2010 cup final for Panathinaikos against Aris Thessaloniki. His club also won the Greek championship. This was the second time that Leto had been part of a club that did the Greek League and Cup double because he had achieved the same feat when on loan from Liverpool at Olympiacos in 2009.

After two average seasons for Panathinaikos he excelled in the 2011/12 season and scored no less than 15 goals in 17 league matches. At the height of his powers he injured his knee in January and missed the rest of the season when Panathiniakos finished runners-up to Olympiacos. He terminated his contract with Panathinaikos in January 2013 after he had still not recovered from his injury. A statement released by Panathinaikos said: “Seba’s talent, his contribution to the team, his passion and the bond he formed with the fans has earned him a special place in the history and the heart of the club.”

After recovering from injury Leto joined Serie A side Catania,  playing in 26 Serie A matches in 2013/14 scoring three times (against Torino, Atalanta and Sampsoria). The Sicilian club was relegated after finishing 3rd from bottom with only 32 points from 38 matches and with the club struggling financially the following season he was loaned to Argentine side Lanus in January 2015. After a year in his homeland he returned to Siciliy, by which time Catania were in the third tier and he was given a free transfer, rejoining Panathinaikos where he scored a respectable five goals from twelve appearances in the rest of the 2015-16 season. He again had a productive season with Panathinaikos in 2016-17 scoring eleven times but two thirds of his appearances were as a substitute. 

Leto spent the first half of 2017-18 with Emirates FC in the UAE Gulf League, playing nine times before leaving. He had talks with Fulham in January 2018 but no deal was struck . In June 2019 Leto announced formally that he was ending his playing days and taking up a coaching role in Iran with Esteghan FC. In 2021 he became assistant manager at Al Gharafa in Qatar before moving to Greece in January 2024 as assistant manager at AE Kifisias. Six months later he was appointed as manager of the Greek second tier club.