Born in Pamplona, Monreal was a product of CA Osasuna's youth system and made his official debut for his hometown club in an away game against Valencia CF on 22 October 2006, a 1–0 La Liga loss.

[7] Monreal became first-choice in 2007–08, retaining that status in the subsequent seasons, with another youth product, César Azpilicueta, being the undisputed starter in the other defensive wing.

[9] In his first season he initially battled for first-choice status with Eliseu, but ended up only missing seven games as the club finished fourth and qualified for the Champions League for the first time.

[18] Monreal returned from injury[19] on 21 August 2013 to feature in Arsenal's 3–0 win at Fenerbahçe SK in the Champions League group stage play-off, playing the last three minutes in the place of Theo Walcott.

[27] On 6 November 2017, after good performances on the left side of a three-man defence,[28] Monreal won the Professional Footballers' Association Player of the Month award for October of that year.

[31] He scored on his competitive debut two weeks later, converting an easy tap-in at the hour mark of an eventual 2–0 victory over Atlético Madrid at Anoeta Stadium.

[37] He played the last 15 minutes for Joan Capdevila in a 3–2 away win;[38] on 7 September 2010, in the nation's second game after they won the 2010 FIFA World Cup in South Africa, he featured the entire 4–1 friendly loss to Argentina.

[39] Monreal was a member of Spain's squad which reached the final of the 2013 FIFA Confederations Cup in Brazil, although he was second-choice left-back behind FC Barcelona's Jordi Alba.