McDonnell is new SDLP deputy leader
Dr McDonnell beat off the challenge from two senior party colleagues to replace Brid Rodgers as Mark Durkan’s number two.
The 54-year-old member of the Stormont assembly was voted in on the second count at the SDLP’s annual conference in Belfast.
Fellow MLAs Alban Maginness and John Dallat had also stood for the post.
Dr McDonnell said his immediate aim was to help rebuild the party which suffered heavy losses to Sinn Fein in November’s Assembly elections.
He said: “This is a major challenge to rebuild where we have slipped a bit.
“We need to build branches and strengthen others that already exist. That’s my agenda.”
He insisted that the SDLP offered the only real way forward for the peace process, and he wanted to be a part of achieving that.