[6][9] In 2008, Greenhalgh was appointed by Eric Pickles, the then Shadow Secretary of State for Communities and Local Government, to head the new Conservative Councils Innovation Unit to formulate new local-government policy, and he was also appointed by the new Mayor of London, Boris Johnson, to oversee a financial audit of the Greater London Authority.

On 6 June 2012,[11] Greenhalgh was appointed, by Boris Johnson, to head the MOPAC, as the second Deputy Mayor for Policing and Crime.

[14] However, the water cannon bought by Boris Johnson for £322k were sold for scrap for just £11k having never been used after the Conservative Home Secretary, Theresa May, banned the police from using them.

[15] In December 2014, Greenhalgh became the third declared candidate for the Conservative Party nomination in the 2016 London mayoral election.

[16] On 18 March 2020 Boris Johnson appointed him as an unpaid joint Minister of State at the Ministry of Housing, Communities and Local Government and the Home Office.

[18] Greenhalgh was the subject of controversy in the first months of his appointment as the Deputy Mayor for Policing and Crime.

He apologised after a colleague stated that he had patted her on the bottom, despite the fact she did not make an official complaint, and that he said he had no recollection of the alleged incident.

Since 2006, he has been a Trustee of the Camelia Botnar Arterial Research Foundation and since 2012 a governor of Hurlingham & Chelsea School.