Armstrong will replace Steve Wyler, who announced in December that he would stand down this year once his successor had been appointed.
Wyler has been head of Locality since it was formed through the merger of the Development Trusts Association, which he had been in charge of, and the community umbrella body Bassac in 2011. He will leave when Armstrong joins Locality in July.
Armstrong has been chief executive of Living Streets, which campaigns on behalf of pedestrians and works to help people improve their local environment, since 2008.
Before joining Living Streets, he was a civil servant for nine years and worked in roles including leading the cross-government obesity programme based at the Department of Health between 2006 and 2007.
He also worked in neighbourhood regeneration and renewal and was one of the original members of staff at the government’s Neighbourhood Renewal Unit in 2001.
Joanna Holmes, chair of Locality, said that Armstrong would bring "something new to the organisation as well as a solid grounding in the kind of work Locality and its members do".
Armstrong said he was looking forward to taking up his new role at an organisation with a "proud history and a great future ahead of it".