Sadiq Khan has appointed Lambeth Council leader Lib Peck to run his Violence Reduction Unit (VRU).
She will quit her council job to become the director of the mayor of London’s new initiative.
Ms Peck said she would be “fully focused” on approaching knife crime as a public health issue.
However, London politicians have expressed “disappointment” at a Labour politician being appointed, rather than a crime professional.
In 2018 the number of homicides in the capital rose to its highest level in a decade.
Announced by Mr Khan in September, the VRU mirrors a similar approach used in Glasgow to tackle serious violence as a disease.
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The mayor said he wanted to replicate the city’s public health approach to violence.
Regarding the appointment of Ms Peck, Conservative assembly member Susan Hall said Mr Khan was “guilty of blatant cronyism”.
She added: “Rather than appointing someone with the skills and experience required to get to grips with London’s crime epidemic, the mayor has chosen to appoint an old political ally.”
Ms Peck rejected this analysis, and said she went through two interviews and a “rigorous assessment”.
She said: “Lambeth is a place which has a high volume of crime and has had some incredibly disturbing incidents of crime.
“As a leader of the council we have had to deal with the reality and complexity of that, so I feel very well versed in the consequences of it.
“We need to make short-term interventions and much more longer-term ones to try and understand the complexities as to why people get caught up in violence.”