Guillaumier began his youth career with
St. Andrews at the age of five. On 26 April 2013, he made his debut for the senior team in a
First Division match against
Gudja United, aged only 15 years and 17 days. In the match he played alongside his uncle, midfielder Joseph Farrugia. In September 2013, after trials with English clubs
Birmingham City and
West Bromwich Albion, he joined the youth academy of Italian club
Empoli, with whom he had trained regularly in the year prior. In 2015, he returned to
St. Andrews, making sixteen appearances in the
Maltese Premier League. In January 2016, he moved to
Birkirkara on a 4.5-year contract, lasting until 2020. On the 29 September 2020 he moved to Ħamrun Spartans for a record fee in Malta of €300,000. In the same season, after winning his first
Maltese Premier League title, he received the award as
Maltese Player of the Year. On 31 January 2022, Guillaumier joined
Siena in the Italian third-tier
Serie C on loan with an option to buy. On 13 July 2023, Polish
Ekstraklasa side
Stal Mielec announced the signing of Guillaumier on a two-year deal, with an option for another season. On 20 June 2025, Guillaumier signed a two-year contract with Greek club
Panserraikos. On 5 January 2026, Guillaumier joined
Ħamrun Spartans for the second time in his career on a six-month contract, reportedly worth €10,000 a month. ==International career==