Section 20 was originally formed as part of the "Section Initiative" within the British military intelligence establishment. The unit was designed to be able to travel around the world covertly neutralising terrorist threats against the United Kingdom and its allies, able to mobilise immediately and have their mobile command station (known as "The Crib") up in a matter of hours. The nature of the unit and its secrecy also allowed the British Goverment to retain a certain level of plausible deniability should an operation abroad turn bad.
In Season 1, the unit was shown to be supervised and based within the Secret Intelligence Service (MI6), with coordination coming out of the MI6 Building in Vauxhall Cross, London. However, in subsequent seasons the unit is shown to be headed by senior government operatives in Whitehall, indicating it is now within the remit of the Security Service (MI5).
On the ground, the unit is headed by a Commanding Officer, who is a member of the British Army often holding the senior rank of Colonel. However, individuals with lower ranks have also headed the unit (such as Lieutenant Colonel Philip Locke and Major Rachel Dalton).
However, the ground team and Commanding Officer are often supervised (albeit not closely) by senior government officials back in London, such as Major Hugh Collinson in Season 1, and Charles Ridley and Crowther in later seasons.
A division of British military intelligence, the ranks of Section 20 are primarily filled with soldiers from various divisions and departments of the British Army.
However, the unit does allow members of other allied military within its ranks, primarily members of the United States Army such as Sergeants Damien Scott and later Samuel Wyatt. Other, non-military officials from the United States have also worked for Section 20, such as DEA Special Agent Kim Martinez. The unit also recruits members from British Commonwealth forces such as the Australian Army in the case of Lance Corporal Gracie Novin.
Although not strictly members of Section 20, the unit has also been shown to liaise closely and conduct operations with organisations such as the Russian FSB (in the cases of Major Nina Pirogova and Captain Katrina Zarkova, and the Royal Malaysia Police (in the case of Inspector Amy Leong).
- Col. Alexander Coltrane, British Army - Alive
- Sgt. Thomas “Mac” McAllister, British Army (former UK Special Forces) - Alive
- Sgt. Samuel Wyatt, US Joint Special Operations Command - Alive
- LCpl. Gracie Novin, Australian Army Special Operations Command- Alive
- Capt. Katrina Zarkova, Russian Federal Security Service Alpha Group - Alive Liaison
- Sgt. Michael Stonebridge, British Army (former Special Boat Service) - Alive
- Sgt. Damien Scott, US Army (former Delta Force) - Alive
- Cpt. Natalie Reynolds, British Army (former Special Reconnaissance Regiment) - Alive
- LCpl. Will Jensen, British Army Intelligence Corps- Deceased
- Col. Adeena Donovan, British Army- Deceased
- SA Kim Martinez, US Drug Enforcement Administration - Alive Liaison
- Maj. Nina Pirogova, Russian Federal Security Service - Alive Liaison
- Maj. Hugh Collinson, British Army, Commanding Officer of Section 20- Deceased
- Lt.Col. Philip Locke, British Army (former SAS), Commanding Officer of Section 20 - Deceased
- Sgt. Julia Richmond, British Army - Deceased
- Maj. Rachel Dalton, British Army (former Special Reconnaissance Regiment), Commanding Officer of Section 20 - Deceased
- Sgt. Liam Baxter, British Army- Deceased
- Col. Eleanor Grant, British Army Intelligence Corps, Commanding Officer of Section 20 - Deceased
- Sgt. John Porter, British Army (former Special Air Service)- Deceased
- Maj. Oliver Sinclair, British Army Intelligence Corps- Deceased
- Cpt. Kate Marshall, British Army Intelligence Corps (former Special Air Service) - Deceased
- Lt. Layla Thompson, British Army Intelligence Corps - Only in Season 1
- Sgt. Danni Prendiville, British Army Intelligence Corps - Only in Season 1