Mark Alan Williams-Thomas (born 9 January 1970) is an English investigative journalist, documentary maker, television presenter, podcaster and private investigator. Before becoming an investigative reporter, he worked as a policeman and family liaison officer with Surrey Police.

Williams-Thomas presented the documentary which is credited with helping expose Jimmy Savile as a paedophile in The Other Side of Jimmy Savile, which led to the Operation Yewtree police investigation against Savile and others including Rolf Harris. He previously worked on a successful investigation against Jonathan King.

Williams-Thomas has also either been involved in or publicly commented on several other high-profile cases, including the disappearance of Madeleine McCann, the Death of Nicola Bulley, the Death of Jay Slater and the PPE Medpro scandal surrounding Michelle Mone and Douglas Barrowman.

Williams-Thomas was a policeman and family liaison officer with Surrey Police from 1989 to 2000 becoming a detective constable for only a year before leaving due to unspecified circumstances. In August 1997 Williams-Thomas was part of an investigation into child abuse material found in the possession of school teacher Adrian Stark, the director of music at St John's School, Leatherhead, Surrey, who committed suicide shortly after his arrest. Williams-Thomas was one of mulitple police offers who worked on an investigation into child abuse by Jonathan King, which led to King's successful conviction.

Between 2001 and 2002, Williams-Thomas was the marketing manager and a director of GumFighters, a "national chewing gum removal specialist". The company were hired by various councils to clean their streets. In 2003, Williams-Thomas was charged with blackmailing a funeral home director, after alleging that there were multiple bodies buried in unmarked graves. An article ran in a national Sunday paper describing the mass burials. He was subsequently acquitted. In 2005, Williams-Thomas set up WT Associates, an independent child protection consultancy firm. In 2007, Williams-Thomas was awarded an MA in criminology from Birmingham City University.

From 2003 on, drawing on his experience in the police force, Williams-Thomas began script advising for various television crime dramas which included BBC series Waking The Dead (2007–2011), BBC series Inspector Lynley Mysteries (2007), Channel 5 series Murder Prevention (2004), ITV series Identity and BBC series The Silence. In August 2012, ITV News broadcast an exclusive interview Williams-Thomas undertook with Stuart Hazell, who was the last person to see missing 12-year-old schoolgirl Tia Sharp alive. Hazell then went missing the day after this interview and was arrested later the same day on suspicion of Sharp's murder. He would go on to be charged with Sharp's murder, with a judge ordering that he serve a minimum of 38 years.

Williams-Thomas presented two Exposure documentaries; Exposure: Predators Abroad and Exposure: Inside the Diplomatic Bag. His undercover work in Cambodia led to the 2013 arrest of a person suspected of offering underage girls for sex and the rescue of two girls, aged 13 and 14. Also in 2013 year, Williams-Thomas presented an ITV program called On the Run, in which Williams-Thomas and his team pursued a convicted child sex offender on the run in Spain.

In 2014, Williams-Thomas covered the verdict of Oscar Pistorius, being the only British journalist to meet with Pistorius during his trial. Pistorius publicly stated his belief that Pistorius was innocent of the murder of his girlfriend Reeva Steenkamp. On 24 June 2016, ITV broadcast Oscar Pistorius: The Interview in which the former Paralympian spoke in a world exclusive to Williams-Thomas, in his first television interview about the night he shot and killed his girlfriend. In 2015, Williams-Thomas investigated the unsolved murder of BBC presenter Jill Dando. Writing in the Daily Mirror he theorized that she was murdered by the London underworld for her work on Crimewatch.