Thurwell joined Central Police in 1984, having been trained at Melton Police College. His first posting was to Jarrold Station. He passed the Detectives Course in 1986 and was assigned to Polk Avenue Station in 1991. In 1992 he was promoted to Detective Sergeant, and was transferred to Hillside Lane Station. In 1995 he was posted to Farham Station, and was promoted to Inspector the same year. He was assigned to the Vice Squad in 1998, where he served as Senior Investigating Officer (SIO) into the investigation of the death of Oliver Stephens-Lloyd.
In 1999 he posted to Filigree Street Station. In 2001 he was again promoted to Chief Inspector, and was assigned to South Ferry Station. In 2003 he was the SIO in the investigation of the controversial death of Lawrence Christopher in police custody.
He retired from Central Police in 2005. During the AC-12 investigation into Sands View in 2015 he was briefly pursued and sought for questioning. However, when the more senior former officer Patrick Fairbank was convicted and imprisoned, the Crown Prosecution Service discontinued their pursuit of him .
Thurwell's name is seen on various pieces of paperwork related to the investigation into child sex abuse at Sands View Boys Home. He was one of the officers assigned to investigate the accusations (although actually covered them up along with CSU Patrick Fairbank), and was SIO in the investigation into the death of Oliver Stephens-Lloyd, which was erroneously determined to be suicide.
It is revealed through information provided to DI Steve Arnott by Jimmy Lakewell that one of the historic police corruption cases Gail Vella was investigating was the death of Lawrence Christopher in police custody in 2003. In a presentation given by DC Chloe Bishop, it is shown that Thurwell was the Senior Investigating Officer at the time, and ensured due to both corruption and incompetence that the perpetrators who originally attacked Christopher were not arrested until weeks after the crime. This allowed the men (Thomas McTulloch, Robert Parker, Michael Harrison and Samuel Bush) to change their appearances, rendering identification and therefore conviction impossible. The case was discontinued by the Crown Prosecution Service and the men were given anonymity.
Another of the attackers was Darren Hunter, the son of Organised Crime Group senior figure Tommy Hunter. In addition to Thurwell, other officers who at the time were also involved in the Lawrence murder enquiry were then DC Ian Buckells and then Inspector Philip Osborne, now serving as Chief Constable of Central Police.
Detectives with Anti Corruption Unit 12 link Thurwell to the case of Oliver Stephens-Lloyd, who was murdered by the Organised Crime Group before he was able to reveal the sexual abuse occurring in the Sands View Boys Home. Thurwell was a Detective Inspector at the time, who worked on the case under then CSU Patrick Fairbank. It is hypothesised by Superintendent Ted Hastings that Thurwell was also working for the Organised Crime Group, and was instructed by Tommy Hunter to sabotage the investigation into Christopher's murder in order to protect his son Darren Hunter.
During her interrogation by AC-12, DSU Joanne Davidson is shown an image of Thurwell and asked if she recognises him, especially given her familial links with Tommy Hunter. Davidson appears uncomfortable and disgusted upon seeing his image, but refuses to answer the question. Superintendent Ted Hastings hypothesises that Thurwell has been involved in her life since she was a teenager, and questions as to whether he is The Fourth Man who assumed control of the Organised Crime Group after Hunter's death. Again, Davidson refuses to answer. She does however seem shocked to discover his connection with the murder and coverup relating to Lawrence Christopher.
Central Police makes contact again with the Guardia Civil in Spain to resume their tracking of Thurwell, as he has once again become a person of interest in Operation Lighthouse. The GC is able to trace him to a remote villa in the countryside, and dispatches the Unidad Especial de Intervención (UEI) to conduct a night raid. Live feed of the raid is streamed to AC-12 through their Cyber Crime Unit, with specialist technician Amanda Yao acting as translator and coordinator due to her fluency in Spanish. The Spanish authorities storm the property and after a brief sweep uncover two dead bodies. It is implied they have been deceased for several days, given the rate of decomposition and number of flies that have been drawn into the property. The Captain leading the raid identifies them as Thurwell and his wife, Mrs V Thurwell.
The bodies found in the villa are formally identified by Spanish authorities as those of Marcus Thurwell, and his wife Mrs V Thurwell. Both bodies show signs of strangulation and the state of decomposition suggests they died a few weeks before the bodies were found. Spanish authorities also found equipment that could be used to route communications via Spain, giving the appearance that they originated from outside the country when in reality the sender was in the UK.
During the AC-12 interview, DSU Ian Buckells refuses to take responsibility for ordering Thurwell's death, suggesting it could have instead been the Organised Crime Group. Superintendent Ted Hastings notes that Thurwell also used encrypted computer software to communicate, however the IP addresses were eventually traced by the Cyber Crime Unit as originating in the UK, not Spain, throwing further evidence to the likelihood that Buckells was indeed the Fourth Man.
- James Nesbitt lied about his role as DCI Marcus Thurwell, stating prior to his appearance that he was upset that he was never asked to be in the show. This lie was cooked up with writer Jed Mercurio to throw viewers off the scent. When seen filming they both simply said Nesbitt was filming a scene for Bloodlands which was filmed in the same area and produced by Jed Mercurio.