Born in Barrio del Progreso, Murcia,[3] Martínez began his footballing career with local side C.F.S.

He made his senior debut in the Segunda División B (third tier) at the age of 16, coming on as a substitute and scoring in a match against AE Prat on 8 December 2013.

[6] In April 2016, Martínez agreed a three-year deal to join English club West Ham United on 1 July 2016 for a fee of around £2.4 million.

[7] He was given permission to train with his new teammates by Valencia to help him settle in England and would initially link up with West Ham United's development squad.

On 28 January, Martínez came on as a substitute to score his first goal for the U's in a 3–0 home win over Newcastle United in the fourth round of the FA Cup.

[13] On 7 January 2018, Martínez made his full West Ham debut in an FA Cup third round tie at Shrewsbury Town, replacing Javier Hernández after 71 minutes in a 0–0 draw.

[14] On 1 February, he made a deadline day loan switch to Segunda División club Real Valladolid for the remainder of the 2017–18 season.

[21] He made his debut on 3 August in the first round of the Taça da Liga, as a 59th-minute substitute for Pedro Gonçalves in a 2–0 home loss to S.C.

[28] He won his first silverware on 23 December in the 2020 Supertaça Cândido de Oliveira, as a last-minute substitute in a 2–0 win over O Clássico rivals S.L.

[35] On 27 August 2024, Martínez returned to his home country after signing a four-year contract with Deportivo Alavés in the top tier.