After starting his senior career at River Plate in 1996, Aimar amassed La Liga totals of 215 games and 32 goals over eight seasons with Valencia and Zaragoza from 2001 to 2008, before spending five years in Portugal with Benfica, winning a combined nine major titles with the three teams.

Aimar's football career begun in earnest when he was offered the chance to play for Estudiantes de Río Cuarto by youth coach Alfie Mercado.

[2] Having turned down the opportunity to attend medical school to pursue a career in football, Aimar made his debut for River on 11 August 1996 against Colón.

[2][5] He made his debut for the club the following month against Manchester United and drew praise from three-time Ballon d'Or winner Johan Cruyff for his performance after the match.

[6] Aimar immediately became a key figure in Valencia's midfield under Héctor Cúper and helped the club reach the 2001 UEFA Champions League final, where they lost to Bayern Munich on penalties.

Aimar appeared in a total of 33 league games and scored 4 goals as Valencia were crowned 2001–02 La Liga champions for the first time in 31 years, ending as the club's third-highest goalscorer across all competitions for the season.

[8] Following a breakdown between Benítez and Valencia's sporting director Jesús García Pitarch, the former left his position at the club and was replaced by Italian manager Claudio Ranieri for the 2004–05 season.

[2][17] On 7 August 2013, Tunku Ismail Ibrahim, the president of Malaysian club Johor Darul Ta'zim, confirmed that Aimar had signed a two-year contract with the team.

[23] Aimar announced his retirement on 14 July 2015 after River Plate coach Marcelo Gallardo informed him that he was not included in the club's squad for the semi-final stage of the Copa Libertadores.

After the match, he announced that he would be joining the squad ahead of the Copa Argentina fixture against Sportivo Belgrano on 23 January, in order to fulfill his wish of making an official appearance for the club where he began and spent eight years.

[26] Aimar was picked in the starting line up and wore the captain's armband, coming close to opening the score with a shot that was parried by the goalkeeper, before getting subbed off in the 50th minute.

In the 2002 World Cup, for which he was picked ahead of Riquelme, Aimar appeared against England, subbing in for Juan Sebastián Verón in the 1–0 loss,[31] which led coach Marcelo Bielsa to start him against Sweden at the latter's expense.

[32] He also appeared with the national side in the 2007 Copa América, scoring a goal in a 4–1 win against the United States in his team's opening group match of the competition;[33] Argentina went on to reach the final of the tournament, losing out to Brazil once again, this time 0–3, with Aimar coming on as a second-half substitute for Cambiasso.

In the match against the former he repaid the faith placed in him by coach Diego Maradona, assisting Gonzalo Higuaín for Argentina's opener with a through ball, in an eventual 2–1 win.

An elegant, technically gifted and creative attacking midfielder with a small frame, Aimar was a right-footed player with a vast array of skills and an eye for goal.

[36][37][38][39][40] He excelled in a free role as an advanced playmaker where he was best able to utilise his dribbling skills, vision and passing ability to create chances for teammates and orchestrate attacking moves.