Smith & Company is headed by Judy Smith, a former advisor to ex-US President George HW Bush and the inspiration for the ABC TV series Scandal for her work in crisis comms.
The London office is the agency's first outside the US. Smith & Company also operates in Washington DC, New York and Los Angeles.
Walkom (pictured) started her role earlier this month, working in the London office with Lucy Collins, a vice president who joined from London & Partners.
Walkom told PRWeek: "We'll be doing a lot to build the business in London over the next while, but it's in its infancy."
She added: "I will be working specifically on a global affairs practice as well. It will be providing strategic advice helping organisations solve problems, manage crises and build and enhance their reputation across all sectors.
"We'll be working very closely with the US team; there are a number of us across different offices and we are one big, cohesive team. We get to use each other's skills and expertise, which makes us stronger and with a bigger wealth of knowledge."
On Smith, whose previous roles include vice president of communications at NBC, Walkom said: "Judy is a woman who inspires me in so many ways: the way she works with clients, the way she provides strategic advice and [she is] is such a value-add to those she works with across a wide range of sectors, including some of the world's most complex issues to straightforward, bread-and-butter issues.
"It's very rare to have an opportunity to work with somebody like that."
She said Save the Children International, which she has been working with on consultancy and client basis for some time, will be looking to recruit her permanent replacement later this year.
Walkom worked for Save the Children in her native Canada before joining the charity's international arm in London in 2015, initially as director of internal comms and later as global comms director.
Before Save the Children, she worked agency-side, including a stint at Edelman in Canada as senior account manager.
Walkom was also fourth in PRWeek's list of most influential people in UK charity comms for the 2019 UK Power Book.