Stock has worked as the events special adviser at Number 10 since Theresa May became Prime Minister in July 2016.
He has previously worked as operations and events manager at the Conservative Party, holding senior operational roles in the 2015 general election campaign, the 2017 London mayoral election and the EU remain campaign.
He has has also worked at Dods Communications, organising commercial events in London and Brussels, and began his career as a public affairs consultant at MHP Communications.
A year ago, Portland appointed three former Downing Street advisers to its senior team: Will Tanner, Laura Trott and Nick Hargrave.
"Clients are increasingly looking for one agency partner to provide 360 degree support from strategy through to delivery," Portland UK managing director Mark Flanagan said.
"We’ve built an events operation that is well equipped to run large or small stakeholder events for clients in any sector. We are delighted to have attracted someone of Jonathan’s calibre to take this important part of our business to the next level."
Stock joins Portland’s team of nine events and marketing specialists who stage events on behalf of clients as part of public affairs and corporate communications campaigns.
"I’m really pleased to join Portland to lead their events team as organisations seek to find innovative and original ways to engage with audiences across the world," he said.
"It’s an exciting time to be in the comms industry and I’m glad that Portland will be where I build on my experience in government."