At 38, Miss Pearce is not only unusual because of her age. The British Foreign Office is regarded as one of the few surviving strongholds of clubby Oxbridge elitism, insulated from the modern world by the thick walls of gracious embassies across the globe. Its new star did not go to Oxbridge, doubts she is clubbable, and is bound for an embassy only recently upgraded from one room in an old East German consulate.
Does she represent a new breed of diplomats - or is she just a lone interloper among Eton's finest? Or is she simply more traditional than her credentials suggest?
"I'm a bit of an odd mess, really," she offers. From a family of teachers, undertakers and butchers in South Wales, Jessica left school after A-levels and became a secretary in Barry. It was, she says, "great fun". Two years on, when she "felt ready" for university, it was to study International Relations in Aberdeen for five years. That was "great fun", too.
There is a common perception that those who make it in Whitehall without the usual advantages are a humourless bunch. Huddled over redbrick university desks late into the night, driven by vague resentments, their ambition is unequalled. When Jessica Pearce graduated, she returned to secretarial work, and passed two years in Cardiff selling wine. It was only when the companylooked like it would stop being "fun", that she thought of the Foreign Office.
"The whole thing is so daunting when you start, if it isn't in your background. At provincial universities it was thought that you had to be brilliant to get in, and that did the Office a terrible disservice, because we thought it was too rarefied to try." She applied, and became one of 20 entries from 2,000 candidates.
Ten years on, after jobs in the East European, African and United Nations departments, and a posting to Dakar, she has her first appointment as ambassador in Minsk.
The sprawling, concrete capital of ex-Soviet Belarus featured in True Brits, the 1994 BBC series about the diplomatic service. Miss Pearce's predecessor, John Everard, was seen setting up a shabby one-room embassy in the old East German consulate, furnished in student bed-sit style. More village cricket captain than emissary of a great nation, Mr Everard was earnestly engaged in delicate negotiations to instate little flags for a visiting British minister.
It is not immediately clear how this is a fitting pursuit for either a country's greatest minds, or its taxpayers' money.
"It's precisely the kinds of changes we saw in the Soviet Union which have made the foreign office a more pragmatic place," she counters. "There's a need for new embassies, and a more flexible approach now, and a sense of value for money."
Certainly, it is easier for a 38-year-old woman to get an ambassadorship in a new outpost like Belarus than at one of the old prestigious embassies like Paris or Washington. The foreign office's most senior woman,Pauline Neville-Jones, broke ranks last month to complain bitterly about having been passed over for the Paris position. She is currently on leave, considering her future.
Miss Pearce's appointment brings the number of senior women to seven, out of some 140 posts. Only one is married, and it is not Jessica. The rule obliging women to resign if they marry was only lifted in 1972; it appears, in practical terms, to persist.
Jessica is strikingly, though perhaps predictably, reluctant to read into these figures or the Neville-Jones affair any negative signals about women's place in the service.
"I'm not at all convinced that she wasn't given the job because she was a woman. If anything, being a woman has been an advantage for me," she says, although when pressed cannot offer examples. Nor does she say that the absence of a wife at her side presents a problem.
"And anyway," she hastens, "I don't think I've consciously taken my gender into account. I'm me as a person and I have a job to do." The Foreign Office itself is entertainingly torn between championing her success and imparting the notion that such figures are increasingly the norm, and Miss Pearce gives no hint of any mission to dismantle its more arcane ways.
"We are on shifting sands, and the world is moving very fast, but there's still room for the great thinking in lofty rooms - for the pondering about whither the world? Whither Britain? Britain has an image to maintain - there is a pillar that is always Britain."
The posting to Belarus is for three years, and she is characteristically vague about what lies beyond.
Jessica Pearce may not represent a new breed of diplomacy, but she does seem to suggest a new and powerful strain. Is she a True Brit? She pauses, polished loafers on the polished marble floor of the Foreign and Commonwealth Office, and considers. "No, I wouldn't say so. I'm Welsh." She clips off across the courtyard to talk to the Bank of England about British interests in Belarus.