Born in London, Mothersille began his career in the academy of Leyton Orient. In January 2020, it was reported that Leyton Orient had agreed a deal with Premier League side Chelsea for Mothersille to join as a scholar the following season.[2] This was confirmed by Chelsea in July 2020, when Mothersille was announced among the new scholars for the 2020–21 season.[3] However, having failed to establish himself in Chelsea's under-18 team, making only one substitute appearance, he was loaned to Derby County to feature for their youth sides on 1 February 2021.[4]
The following season proved more successful at Chelsea for Mothersille; having scored consecutive braces in the EFL Trophy against former side Leyton Orient and Peterborough United, Mothersille was called up to Chelsea's first team squad for a mid-season, warm-weather training camp in Abu Dhabi, and he featured as a substitute in Chelsea's 1–0 friendly loss to Aston Villa - his first and only appearance for the club.[5]
With his contract set to expire at the end of the 2022–23 season, it was reported by the Evening Standard that Mothersille was close to agreeing a new deal with Chelsea in January 2023.[6] However, by May, he had still not accepted Chelsea's offer,[7] and in August, with his contract having expired, he went on trial with fellow Premier League side Everton.[8]
Peterborough United
On 2 September 2023, League One side Peterborough United announced that they had signed Mothersille on transfer deadline day, with an undisclosed compensation fee being agreed with Chelsea.[9][10]
^Johnson, Simon (15 December 2022). "With Armando Broja injured, which young forward will step up for Chelsea?". theathletic.com. Retrieved 3 September 2023. scored successive braces against professional opposition in the Papa Johns Trophy, firstly against former club Orient and then versus League One Peterborough. Significantly, [Graham] Potter then gave Mothersille more minutes than Burstow in that weekend friendly against Villa, bringing him on with 26 minutes remaining.