Casillas began his career in Real Madrid’s youth system, known as La Fábrica, during the 1990–91 season. On 27 November 1997, at age 16 and still a junior, he was first called up to the senior team squad to face Rosenborg in the UEFA Champions League, although he remained on the bench throughout. After spending a season with the C-team in the fourth tier, during which they won their regional group, he came into contention as the club’s first-choice between the posts.
In starting the 1999–2000 UEFA Champions League group stage fixture against Olympiakos on 15 September 1999, he became the youngest goalkeeper ever to feature in the competition at the time, aged 18 years and 177 days; a record which was only broken in October 2017 by Mile Svilar. Three days earlier, Casillas had made his La Liga debut in a 2–2 draw against Athletic Bilbao at San Mamés Stadium. In May 2000, he became the youngest ever goalkeeper to play in and win a Champions League final when Real Madrid defeated Valencia 3–0, just four days after his 19th birthday.
Casillas became the first choice goalkeeper at Real Madrid, winning two Champions Leagues and La Liga titles in his first three seasons, establishing himself as one of the best goalkeepers in the world. During his highly successful career in Madrid, Casillas won all major club titles, including five La Liga titles, four Supercopa de España titles, two Copa del Rey titles, three UEFA Champions Leagues, two UEFA Super Cups, one Intercontinental Cup and the FIFA Club World Cup.
After 25 years with Real Madrid, Casillas joined FC Porto in 2015 on a free transfer, where he won the Primeira Liga and a Supertaça Cândido de Oliveira title. While at Porto, he broke the record for most consecutive seasons played in the UEFA Champions League (20) and amassed the most appearances in the competition (177).
In January 2012, Casillas agreed a sponsorship deal with Adidas to wear their Response goalkeeping gloves and Predator football boots. The move to Adidas ended a long running association for Casillas with Reebok, which started in 2004. In February 2005, Casillas was the football face of Reebok’s “I Am What I Am” global integrated advertising campaign that linked all the brand’s marketing efforts under one umbrella. Adidas has been the parent company of Reebok since 2005.
With 167 international caps, Casillas has the second most appearances for the Spain national team, making him the joint tenth-most capped footballer of all time. An unused substitute at UEFA Euro 2000, Casillas became the nation’s first choice goalkeeper at the 2002 World Cup, and went on to play at UEFA Euro 2004 and the 2006 World Cup. In 2008, he was made captain, and went on to lead the team to the European Championship title that year.
Casillas also led Spain to their first World Cup win in 2010, where he kept a joint–record five clean sheets, winning the Yashin Award for the best goalkeeper of the tournament. Spain became the first nation to retain the European Championship, winning the title again in 2012, where he set the record for most consecutive minutes without conceding a goal in the competition (509). In 2014, Casillas became one of four players to represent Spain at four World Cups.
In July 2019, following his heart attack two months earlier, Casillas returned for pre-season training. Later that month, Porto announced that he had been given a role with the club’s coaching staff as he continued his recovery. In August, he was named in Porto’s squad for the 2019–20 season.
In September, he shared the results of his medical tests on social media; at the end of the month, he stated that he would wait until March of the following year to see how he is progressing with his recovery before deciding whether to return to playing or retire. In October, he posted a photo of himself working out in the club’s gym on Instagram. He resumed training on 4 November.
On 4 August 2020, Casillas officially announced his retirement from professional football. In July, he was reported to have returned to Real Madrid to work as an advisor for Florentino Pérez. In December 2020, he was appointed as the deputy to the Real Madrid Foundation CEO.