A 250-year-old census came to light during cataloguing by the National Register of Archives for Scotland (NRAS).
The census lists 90 people living on the remote archipelago on 15 June 1764 - 38 males and 52 females, including 19 families and nine individuals.
Until now, the earliest record dated from 1822.
The islands, which lie about 40 miles west of Benbecula in the Outer Hebrides, were home to generations of people until the last were evacuated in 1930.
The last surviving former resident of St Kilda, Rachel Johnson, died earlier this year at the age of 93.
The census was discovered among the papers of Clan Maclachlan by researchers from NRAS - the branch of the National Records of Scotland which holds historical papers held in private hands in Scotland.
It is not known exactly why the census was taken, or by whom, but NRAS experts said it was likely to have been part of a wider report on the Hebrides.
As the later document from 1822 included ages, it has been possible to track five residents of St Kilda who appeared on both censuses.
The 1764 census also includes the ancestors of the final five families to be evacuated from the island in 1930 - the MacQueens, Fergusons, Gillies, MacDonalds and MacKinnons.
As well as the names of the 90 inhabitants, the document says that they each ate "36 wild fouls eggs and 18 fouls" (seabirds) a day - an overall daily consumption of 3,240 eggs and 1,620 birds.
According to exports, birds and birds' eggs were the mainstay of the islanders' diet.
As well as providing food, fulmar oil was used to light lamps, while seabirds' down was used for bedding and their fat for healing salves.
Dr Alison Rosie, registrar of the National Register of Archives for Scotland, said: "This document sheds new light on the history of St Kilda and the families who lived there, and gives us an insight into their lives more than 250 years ago."
Scotland's Culture Secretary Fiona Hyslop said: "Many people are fascinated by the history of St Kilda, and this census gives us a new insight into the history of the island and its people, which will now be available to researchers thanks to the work of National Records of Scotland.
"Discoveries like these add great depth to our culture and heritage, helping us to understand more about our nation's story."
Earlier this year, lost songs from the evacuated archipelago were discovered and brought to life on a new album featuring renowned composers including Sir James MacMillan.

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Researchers have discovered the oldest known record of the population of St Kilda.
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