Jennings began broadcasting on the pirate radio station Q102 in 1985. When the station closed down in 1988, he was part of the final broadcast. When Millennium 88FM opened to celebrated the first millennium in Dublin in 1988, he was a member of the news team.

When the station closed down in 1989 he moved to Millennium 88FM's parent RTÉ Radio. Jennings worked as journalist and newscaster for RTÉ 2fm. He then moved formally in to RTÉ News and Current Affairs. He is presently the morning newscaster on RTÉ Radio 1, beginning at 5:30 am on RTÉ Radio, and on Morning Ireland.

Former RTÉ Radio 1 broadcaster Gay Byrne praised Jennings' pronunciation of words. Writing in The Irish Times in 2004 he named Jennings along with three other RTÉ journalists who do not mispronounce words. He was present at the 25th anniversary celebrations for Morning Ireland in November 2009.

The Irish Times journalist Michael Dwyer was Jennings' partner for 24 years. Dwyer died in 2010. At the funeral fellow RTÉ journalist Aengus Mac Grianna read out a personal tribute from Jennings. Jennings' father also spoke at the funeral.

After Dwyer's death, Jennings presented an award at the Jameson Dublin International Film Festival in his honour in March 2010. The first "Michael Dwyer Discovery Award" was given to Kate McCullough.