Jennings was born in Scotland and as a child moved to London, England. He is the grandson of a former
Clapton Orient player.
 Jennings attended University of Hull
 and later worked for the
Sunday Times' Insight team in the late 1960s, after which he worked for other British newspapers before becoming an investigative reporter on
BBC Radio Four's Checkpoint. In 1986 the BBC refused to broadcast his documentary concerning corruption in
Scotland Yard; Jennings reacted by resigning and transforming the material into his first book, Scotland Yard's Cocaine Connection, and the documentary was aired by
World in Action.
Jennings subsequently worked for
Granada, filming several international investigations and small documentaries. His investigation of British involvement in the
Iran-Contra affair won the gold medal at the 1989 New York TV Festival. In 1993, Jennings entered
Chechnya with the first western TV crew ever to enter the country, to investigate
Caucasus mafia activity. 1997 saw Jennings working with World in Action, with an investigation on British Olympic swimming coach
Hamilton Bland, and in 1998 he presented a documentary on rail
His first appearance on
Panorama, a current affairs documentary programme, came in June 2006 (episode entitled "The Beautiful Bung: Corruption and the World Cup") when Jennings investigated several allegations of bribery within
FIFA, including million-dollar bribes to secure marketing rights for the body's
sports marketing company
ISL along with vote-buying (to secure the position of FIFA president
Sepp Blatter), bribery and graft attributed to
 It was followed up in October 2007 with an episode entitled "FIFA and Coe" exploring the relationship between former British Olympian
Sebastian Coe and the
FIFA Ethics Committee.
The most prominent programme was
FIFA's Dirty Secrets (first aired on 29 November 2010) which was a 30-minute investigation of corruption allegations against some of the FIFA executive committee members who were to vote on the host for the
2018 FIFA World Cup. Jennings alleged that
Ricardo Teixeira, President of Brazil's Football Federation (CBF) and of the 2014 World Cup Organising Committee,
Nicolás Léoz of Paraguay, President of the South American Football Confederation (CONMEBOL), and
Issa Hayatou from Cameroon, President of the
(CAF) all accepted bribes from a television marketing firm.
In December 2015, he presented a summary of the investigations into FIFA entitled Fifa, Sepp Blatter and Me for BBC's Panorama.