Guilherme Martinho Peixoto was born in Avanca, in the Aveiro district, on 25 May 2006. A right-back, he joined Benfica's academy in 2019 and progressed through the under-15, under-17 and under-19 sides, signing a professional contract in October 2022 and renewing it after a 2023/24 season in which he made 27 appearances for the junior team and played in the UEFA Youth League.

In 2024/25 he was promoted to the under-23 squad, playing around 25 matches in the Liga Revelação, and was registered with the number 96 in the Benfica B squad. No competitive appearance for the B team in Liga Portugal 2 is recorded for him.

His contract was not extended beyond 2025 and on 3 July 2025 he left on a free transfer for FC Twente, signing until 2028 with the option of a further year. He made his Eredivisie and KNVB Beker debuts in 2025/26. He has been capped by Portugal from under-16 to under-20 level.