Born in Justiniano Posse, Córdoba, Demichelis started playing professionally in 2001, with Buenos Aires-based River Plate, having arrived at the club's youth system three years earlier.

On 28 April 2002, River Plate goalkeeper Ángel Comizzo was sent off with a minute of regulation time remaining against Racing Club, leaving Demichelis to take over in goal.

He faced a direct free kick that hit the wall and was countered for the only goal of the game by Nelson Cuevas, as River won the Clausura title.

[7] Under new coach Felix Magath, Demichelis became a regular starter for Bayern, appearing in 75 official matches in two seasons combined as the team won back-to-back doubles, achieving a total of four during his spell.

[9] On 29 October 2010, benefitting from a run of injuries in the squad, Demichelis made one of his last appearances for Bayern, heading a goal in a 4–2 home win against SC Freiburg.

[11][12] He rejoined former River Plate manager Manuel Pellegrini, and made his La Liga debut on 8 January 2011, scoring in a 1–1 home draw against Athletic Bilbao.

[32] He made his debut under José Pekerman on 12 November 2005 that year, in a 3–2 friendly loss to England at the Stade de Genève in Switzerland, playing as a defensive midfielder;[33][34] he was not chosen for the 2006 FIFA World Cup.

[36] He suffered a facial injury during an exhibition game with Germany on 3 March 2010, being sidelined for three weeks,[37] and later being selected for the year's World Cup in South Africa.

Under Diego Maradona, he played in all of Argentina's games at the tournament, partnering Nicolás Burdisso in central defence in an eventual quarter-final exit; on 22 June, he scored the first goal in the 2–0 group stage win against Greece.

[40] On the bench for the team's first four games, Demichelis made his debut in the tournament in the 1–0 quarter-final win over Belgium in which he played the full 90 minutes, taking the place of Federico Fernandez.

[41] He went on to start in the semi-finals where his team eliminated the Netherlands in a penalty shootout after a 0–0 draw, and again in the final against Germany and picked up the runners-up medal as Argentina lost 1–0 after extra time.

[42][43][44] Demichelis was chosen for the 2015 Copa América in Chile, making his debut in the final group game, a 1–0 win over Jamaica, as a starter in place of Nicolás Otamendi.

[46][47] In May 2017, immediately after retiring, Demichelis remained at Málaga as assistant manager to Míchel, replacing Weligton who had returned to Brazil due to injury.