She leads a team of about 15 in the role, which she took up earlier this month, and will report to Janine Lloyd-Jones, the department's deputy director for strategic communication and campaigns.
Hallett joins from the Department for Environment, Food and Rural Affairs, where she was head of comms strategy and campaigns.
A psychology graduate, she has enjoyed a rapid ascent since starting in public-sector comms in 2015. Prior to that she held a succession of roles in PR agencies such as Kindred, MGB PR and Warwick Emanuel, as well as completing a short stint at the RSPB charity and 16 months at UBS Investment Bank in the US.
Most recently, Hallett spent two years at Defra as a senior strategic comms manager.
She told PRWeek that the "international dimension" of the FCO, with its "huge virtual network" of posts overseas, played a big part in her move.
In terms of immediate priorities, Hallett will be working on a campaign about media freedom, with the FCO set to host a conference on this issue in London this summer.
Hallett describes herself as a "seasoned communications practitioner, who takes great pride in building long-standing and trusted relationships with clients, stakeholders and colleagues".
She recalls how when she joined Defra, "the first thing that struck me was how GCS has professionalised government communications".
It is something the private sector could learn from, she said.
Hallett has an "element of commerciality" from her time in the private sector and is keen to work closely with commercial organisations on the FCO’s campaigns.
"I was really keen to pull the outside in on our campaigns at Defra and I plan to do this at the Foreign Office as well," she said.
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