[1] Born in Sheffield, Yorkshire, Brennan began his career as a runner and then a news photographer for The Croydon Times.
[2] He first gained notice for a series of photographs he took of the death of Donald Campbell who died while attempting the world water speed record in 1967.
After then moving to the Daily Mail he covered important news events like The Troubles in Northern Ireland and The Indo-Pakistani wars and conflicts and spent time with the then unknown Mother Teresa of Calcutta.
During his career Brennan has photographed the following individuals:[6] Queen Elizabeth II, Nelson Mandela, Princess Diana, Pelé, George Best, Jimmy Carter, Mick Jagger, John Lennon, Norman Mailer, Frank Sinatra, Barbra Streisand, P.G.
Fidel Castro, Tony Blair, Paul McCartney, Rod Stewart, Freddie Mercury, Joe Namath, Jimmy Page, Alice Cooper, Elvis Presley, Mickey Rourke, Robert Mitchum, Mother Teresa, Artur Rubenstein, Mike Tyson, Henry Miller, Elmore Leonard, John Grisham, E Howard Hunt, Elizabeth Ray, Tommie Smith, Liv Tyler, Jacqueline Kennedy-Onassis, George Wallace, Clara Hale, Deborah Harry, George Foreman, Joe Frazier, Marvin Hagler, Lennox Lewis, Ricky Hatton, Björn Borg, Andre Agassi, Alger Hiss.