First elected as a councillor in Camberwell Green ward in 2014, Cllr Williams has served in Southwark's cabinet for the past two years, holding the jobs portfolio in 2018/19 and currently having responsibility for housing management.
Before holding elected office Cllr Williams' career included stints with Sheffield City Council and Lambeth Council where he was head of health and wellbeing.
Cllr Williams won the leadership in an election contested by four of his councillor colleagues: Johnson Situ, Sunny Lambe, Martin Seaton and Stephanie Cryan.
The formal handover from outgoing leader Cllr Peter John takes place at Council Assembly on Wednesday 16 September.
Cllr John had been due to step down in March this year but delayed his departure due to the COVID-19 crisis.
Cllr Jasmine Ali was chosen by the Labour group as deputy leader.
This year the leadership of both political groups on Southwark Council has changed hands: in the spring Cllr Nick Johnson took over from Anood Al-Samerai as leader of the opposition Lib Dem group.
Labour holds 49 seats on the council to the Lib Dems' 14.