McDevitt, who joins Hume Brophy as associate partner, was the SDLP's spokesperson on health and policing and the member of the legislative assembly (MLA) for South Belfast.
He resigned on 4 September after failing to declare £6,750 he received between March 2010 and August 2010 from Weber Shandwick after his election as an MLA, saying: "I feel I have fallen below the standards expected of someone in public life."
He said the payments were in return for providing support and mentoring to the agency team that took over from him after he left his role as managing director of its Belfast office in December 2009.
A Hume Brophy spokeswoman confirmed it was aware of the issue when it hired McDevitt. She said: "The information was in the public domain and the matter was dealt with. As far as were concerned that’s that."
Managing partner John Hume said: "Conall brings to our company 20 years of experience in the public and private sectors. He has a unique insight into public policy and business in the UK, Ireland and Europe."
McDevitt has stated that he never represented, or was asked to represent, Weber Shandwick's clients while an MLA.
His previous roles include director of comms for the SDLP from 1996 to 1999, a time that included the negotiations running up to the Good Friday Agreement, subsequent referendum and assembly elections.