In the 1931 Birthday Honours, he was awarded the King's Police Medal (KPM)[5] for bravery after he confronted Indian independence activist Chandrashekhar Azad on 27 February 1931.

On 29 June 1933, Nott-Bower joined the Metropolitan Police as Chief Constable (second in command) of No.1 District, consisting of A (Whitehall), B (Westminster), C (St James's), T (Hammersmith) and V (Wandsworth) Divisions.

In his 1955 book Against the Law, Peter Wildeblood quotes an article written by Donald Horne for the Sydney Morning Telegraph printed on 25 October 1953 referring to Nott-Bower's role in the 'Great Purge' .

"The plan originated under strong United States advice to Britain to weed out homosexuals – as hopeless security risks – from important Government jobs.

One of the Yard's top-rankers, Commander E. A. Cole, recently spent three months in America consulting with FBI officials in putting finishing touches to the plan.

But the plan was extended as a war on all vice when Sir John Nott-Bower took over as the new Commissioner at Scotland Yard in August.

Under laxer police methods before the US-inspired plan began, and before Sir John moved into the top job at the Yard as a man with a mission, Montagu and his film-director friend Kenneth Hume might never have been charged with grave offences against Boy Scouts.... Sir John swung into action on a nationwide scale.

He did set up the Research and Planning Branch and the Metropolitan and Provincial Regional Crime Squad and centralised traffic control in response to rising private car ownership.

He did little to combat the rising crime rate, however; he refused to address the outdated hardline attitudes of many senior detectives, which were becoming increasingly out of step with postwar society; and he did not support his men in their claims for better pay and conditions.