In 1989, Dennis became a presenter, reporter and producer at Sky News, before moving to South Africa in 1991, freelancing for the BBC World Service, mainly covering the final days of Apartheid South Africa.

In 1994 she became the presenter of Carte Blanche, South Africa's top weekly current affairs feature programme. In 1995, she returned to London to work briefly as a correspondent for BBC1's Here and Now weekly programme. Later she became a main presenter of BBC World News, presenting the first edition of The World Today when it was launched in September 2001. Dennis remained as the main presenter of The World Today until she left the BBC in 2014.

Commenting on her new role, she says, “My journalistic career has allowed me the opportunity to cover some of the biggest broadcasting events in the world. In my eyes, Al Jazeera has always been a media organisation which reports and covers the key events with courage, tenacity and through innovative ways. I hope by joining Al Jazeera I can not only bring my experience as a journalist but also be part of Al Jazeera's success as it continues to report as an innovative and invigorating news channel.”

Al Anstey, managing director of Al Jazeera English, says, “We are delighted to welcome Martine to Al Jazeera English. She brings huge experience, great journalism, and a knowledge of the world that is a perfect fit for Al Jazeera.”