Born in Sant Joan Despí, Barcelona, Catalonia,[3] Maffeo joined RCD Espanyol's youth setup in 2003, from FC Levante Las Planas.

[4] On 7 April 2013, he made his senior debut for the reserves at the age of only 15, coming on as a late substitute in a 1–0 home win against CE Constància in the Segunda División B.

Maffeo was called up by first-team manager Manuel Pellegrini for a match against Watford on 29 August 2015,[7] but remained unused in the 2–0 home win.

[9] Maffeo made his competitive debut for City 24 August 2016, starting in a 1–0 victory over FC Steaua București in the play-off round of the UEFA Champions League.

[11] On 13 January 2016, after signing a new three-year contract with City, Maffeo joined Segunda División club Girona FC on loan until June.

[28] On 10 November 2023, he was called up by Argentina manager Lionel Scaloni for games against Uruguay and Brazil in the 2026 FIFA World Cup qualifiers, being eligible to play for the latter country through his mother.