*Club domestic league appearances and goals, correct as of 04:22, 18 August 2024 (UTC) ‡ National team caps and goals, correct as of 23:33, 27 June 2023 (UTC)
He came through the youth ranks at Valencia, where he made his first-team debut in 2020. He made his senior international debut for Spain in 2021 and was chosen for the 2022 FIFA World Cup.
On 23 December 2017, Guillamón renewed his contract with the Che until 2020.[4] The following 27 October he scored his first senior goal, netting the opener in a 3–2 home loss against CD Alcoyano.[5]
Guillamón made his first team – and La Liga – debut on 22 February 2020, replacing injured Eliaquim Mangala at half-time in a 3–0 defeat away to Real Sociedad.[3][6] On 22 July, he renewed his contract until 2023, being definitely promoted to the main squad.[7] He scored his first goal for the team on 22 November in a 2–2 draw at Deportivo Alavés.[8]
On 13 August 2021, in the first game of the season at home to Getafe CF, Guillamón fouled Nemanja Maksimović after 39 seconds. After consulting the video assistant referee, his third-minute dismissal was the earliest on a La Liga opening weekend; his team nonetheless won 1–0.[9]
Guillamón started in the 2022 Copa del Rey Final, being substituted in the 85th minute as Valencia lost on penalties to Real Betis.[10] On 3 October that year, he extended his contract to last until June 2026.[11]
Due to the isolation of some national team players following the positive COVID-19 test of Sergio Busquets, Spain's under-21 squad were called up for the international friendly against Lithuania on 8 June 2021.[14] Guillamón made his senior debut in the match and scored in the 3rd minute of their 4–0 win.[15]
Guillamón was chosen for the 2022 FIFA World Cup in Qatar.[16] He took no part in Spain's four matches, and the side were eliminated in the last 16 by Morocco.[17]