Catherine Sinclair was born in Edinburgh on 17 April 1800, the fourth daughter of Sir John Sinclair, 1st Baronet and Lady Diana Macdonald. Catherine died unmarried. Sir George Sinclair, 2nd Baronet, John Sinclair (1797–1875), and William Sinclair (1804–1878) were her brothers. She was her father's secretary from the age of 14 until his death in 1835. From 1814 to 1818 she lived at Ormeley Lodge, Ham. She was an aunt of the novelist Lucy Bethia Walford.
Sinclair then began to write independently, her first works being children's books, prompted by an interest in her nephew, the Hon. George Boyle, 6th Earl of Glasgow. Her story of two anarchic children, in Holiday House, A Book for the Young, successfully engaged the imagination of her young readers. This work was a popular and notable example of the genre, for it departed from the moralising approach of other works for children in that period. It also encapsulates a fantasy tale of fairies and giants.
On the subject of children's literature, Sinclair remarks in her preface,
"But above all we never forget those who good humouredly complied with the constantly recurring petition of all young people in every generation, and in every house, — 'Will you tell us a story?'"
Sinclair's activities in Edinburgh included charitable works such as the establishment of cooking depots in old and new Edinburgh, and the maintenance of a mission station at the Water of Leith. She was instrumental in securing seats for crowded thoroughfares, and she set the example in Edinburgh of instituting drinking fountains, one of which bore her name and stood at the city's West End before it was removed as an obstruction to trams in 1926. Sinclair is also noted as being the discoverer of Sir Walter Scott's authorship of "The Waverley Novels" which were originally written anonymously.
Catherine Sinclair died at the vicarage, Kensington, the residence of her brother, Archdeacon John Sinclair on 6 August 1864, and was interred in the burial ground of St. John's Episcopal Church, Edinburgh. Her portrait was drawn in crayons by James Archer, R.S.A. (cf. Cat. Third Loan Exhib. No. 620).
A monument, in the style of an Eleanor cross and designed by David Bryce, was erected to her memory on the SE corner of St Colme Street in Edinburgh's New Town (just north of Charlotte Square). The inscription is,
"She was a friend of all children and through her book 'Holiday House' speaks to them still."
The monument was modelled loosely on the Scott Monument, to which she had been a major contributor of funds.
Miss Sinclair wrote brightly and wittily, and displayed much skill in characterisation and description. Several of her books were popular in America.
Undated and early works of Miss Sinclair's are: Charlie Seymour; Lives of the Cæsars, or the Juvenile Plutarch; Holiday House: A Book for the Young (written in 1839 and once very popular with children); Modern Superstition; and Memoirs of the English Bible.
Her other principal works are:Modern Accomplishments, or the March of Intellect, a study of female education. 1836
Shetland and the Shetlanders, or the Northern Circuit. 1840
Scotland and the Scotch, or the Western Circuit. 1840 (republished in America, and translated into various languages)
Modern Flirtations, or a Month at Harrowgate. 1841
Scotch Courtiers and the Court. 1842
Jane Bouverie, or Prosperity and Adversity. 1846
The Journey of Life. 1847
The Business of Life. 1848
Sir Edward Graham, or Railway Speculators. 1849
Lord and Lady Harcourt, or Country Hospitalities. 1850
The Kaleidoscope, or Anecdotes and Aphorisms. 1851
Beatrice, or the Unknown Relatives. 1852
Popish Legends, or Bible Truths. 1852
London Homes. 1853
Cross Purposes. 1853
The Cabman's Holiday. 1855
Torchester Abbey. 1857
Anecdotes of the Cæsars. 1858
Sketches and Short Stories of Scotland and the Scotch, and Shetland and the Shetlanders. 1859
Sketches and Short Stories of Wales and the Welsh. 1860