William Still was born on 14 October 1992[1] in Braine-l'Alleud, Belgium to English parents, Jane (née Bagley) and Julian Richard Still, who left the United Kingdom two years earlier.

[3][4] Growing up in the Walloon region, near Brussels, he went to a French language school and then learned Dutch by playing for Flemish football clubs.

[8] In April 2017, Still started a new position at second division team Lierse, combining the video analyst job with the assignment as assistant manager of Frederik Vanderbiest.

[9] In June 2017, Still returned to Standard Liège,[10] only to leave two days later as his new club did not want to fulfill the agreed upon arrangements as part of the contract negotiations.

[11] Still was able to return immediately to Lierse, where (at just 24 years of age) he was appointed caretaker manager in October due to the sacking of Vanderbiest.

Since he did not hold a UEFA Pro Licence, Reims was fined €25,000 for every match Still managed[19] until he started a monthly course at the National Football Centre near Brussels.

[20] He started his tenure as head coach at Reims with a 14-game unbeaten streak in all competitions which ended with a 3–1 defeat against Toulouse in the Coupe de France round of 16,[21] including two draws against league leaders Paris Saint-Germain.

[18] With a 1–0 win against Monaco on 12 March 2023, Still extended his unbeaten start as Reims manager to 17 matches, setting a new Ligue 1 record.

He also became just the second manager to reach this mark in top-five European leagues in the 21st century, following Tito Vilanova with 18 matches at Barcelona in 2012–13.