As a rising star of the English Bar, Mayne applied in 1829 to be one of the Joint Commissioners of the new Metropolitan Police and was selected without interview.

Later that month, they moved into their offices in 4 Whitehall Place and set about the monumental task of creating the new police force from nothing.

Mayne was responsible for the second section of the General Instruction Book, which laid down the legal standing and powers of a police officer and the law he was required to enforce.

It was a mutual dislike, and although Rowan was more tactful, the Metropolitan Police and Home Office were at odds for sixty years.

However, Mayne's policing at the Great Exhibition was so successful that he was finally appointed Knight Commander of the Order of the Bath (KCB).

He embraced the new Victorian 'morality' introduced by Prince Albert and instructed his men to enforce regulations that were seen by many as petty and unnecessary (such as forbidding children to throw snowballs in public places).

In fact, in many ways his new attitude was conflicting with the instructions written by him as a younger man; now the police were very much enforcing middle-class morality and were treating the gentry and aristocracy with a deference that sometimes interfered with their duties.

In 1866, Mayne took personal charge of suppressing the Hyde Park demonstration, and lost control, suffering physical injury himself.

He was survived by his widow, Georgina Marianne Catherine, eldest daughter of Thomas Carvick of Wyke Manor, Yorkshire, whom he had married in 1831, and children including his son, Rear-Admiral Richard Mayne of the Royal Navy.