Duncan Ball joined the Metropolitan Police Service in 1989. He completed several local policing posts before transferring into Specialist Operations in 1995.

From the mid-1990s until 2014, Duncan served within the Counter Terrorism Command at all ranks up to and including Detective Chief Superintendent and Temporary Commander. During this time, he led a number of major counter terrorism investigations as a Senior Investigating Officer. In June 2014, Duncan was promoted to Commander and appointed to the role of Commander Gangs & Organised Crime responsible for the Serious & Organised Crime Command and the Trident Gangs Command. He held responsibility for targeting Organised Crime Groups involved in firearms, drugs supply, major crime, cyber-crime and modern slavery.

In 2017, he took the National Police Chiefs Council (NPCC) lead in tackling gangs where he has maintained a strong focus on promoting national coordination to target the growing trend of County Lines. He has also recently taken the NPCC portfolio lead for knife crime.

In November 2017 Duncan was promoted to the rank of Deputy Assistant Commissioner and currently holds the MPS portfolio for Met Operations - Covert Policing and Intelligence.