In his new role Harper, who joined Open City after working as deputy director of the Architecture Foundation for five years, wants to encourage people to be more involved in decisions that impact their cities.
"I believe a good city is one in which all its citizens have a voice in how it is made," he told Dezeen.
"Open City has for years been a pioneer of diversity in the built environment, creating educational programmes which expand who is able to study architecture and create the city we live in. All organisations who want to help create better cities must now have diversity at the heart of their agenda and practice."
"I hope to help Open City lead debate about London's future"
In London, where Open City is based and the Open House London event opens over 800 buildings to the public each year, Harper wants to organisation to lead discussions about the city's future.
"I hope to help Open City lead debate about London's future, making the propositional case for a truly equitable, adventurous and open city," he told Dezeen.
"Buffeted by the climate crisis and political change, I believe the UK faces an uncertain future intrinsically connected to the key question of its openness. Simultaneously, it is without doubt that cities and towns, rather than nation-states, will be the centres of ambitious social and economic thinking and innovation in the coming decades," he continued.
"Against these intersecting challenges, Open City is at the centre of some of the most dynamic discussions in urban thinking today."
Open City's "strength is the incredible volunteers"
Harper hopes that Open City's huge network of volunteers can be encouraged to collaborate on other ethical projects that benefit the city.
"Open City's unique strength is the incredible volunteers who give their time to organising the Open House festival every year. It is an extraordinary event and testament to what is possible when like-minded people come together behind a meaningful project," he explained.
"I wonder what else Londoners who care about ethical city-making, from young architects to veteran planners, could accomplish by using the Open City network to collaborate on something together that would be impossible alone."
Beyond London, Harper wants the organisation to help connect people with architects, planners and designers to help create more open cities.
"Over a quarter-century, the Open House Worldwide network has given citizens from Lagos to London greater ownership and access to their cities," he explained.
"In the crucial years ahead, our network of 40 sister organisations has a key role to play connecting people with policy-makers and urban practitioners exploring what real openness should mean in the twenty twenties."
Harper replaces former Architects' Journal editor Rory Olcayto as director. He will take up his position at the end of March 2019.
Open City was established in 1992 and is best known for running Open House London, which it describes as the largest architecture festival in the country.