The Duke of Montagu was a notorious practical joker, but also a liberal and tolerant man with wide interests.  He saw Sancho out and about and brought him home to amuse his wife, Lady Mary Churchill, but also took an interest in him and encouraged him to learn to read and write.  Montagu died in 1749, and this seems about the time Sancho decided he could no longer live with the sisters, and tried to find a place with the bereaved Duchess.  It appears at first that she sent him away, but was later persuaded to employ him as a butler.  When she died in 1751, Lady Mary left him with a year's salary and a £30 annuity.  Sancho promptly fell into women and cards.  However, after an 'unsuccessful contest at cribbage with a Jew, who won his cloaths', he appears to have given up gambling.

What Sancho did between 1751 and 1766, when he re-enters to the service of the Montagu family, is a bit of a mystery.  He returned as a valet to his old employer's son-in-law, who had inherited the title (in a roundabout way: it was recreated for him).  In 1768, he was painted by Thomas Gainsborough as above.  He married Ann Osborne, a young woman of 'West-Indian origin', and probably also in the service of the Montagus in some fashion.  By 1773 Sancho was crippled by gout and could no longer work for the Duke, who accordingly, set him up with a freehold in Westminster and a small grocery shop, which appears to have been successful enough to keep him and his family. 

He was a prolific letter writer, and some of them show an astonishing, journalistic writing style.  In particular, the ones sent to John Spink giving a detailed account of the Gordon Riots, with times noted next to actual events, is invaluable.  His letter of 1766 to Laurence Sterne, the master of sentimentality, gives a neat picture of his life, devoid of self-pity:

He wrote to newspapers under the pseudonym Africanus, and positively identified himself as of 'Afric' birth.  In 1776, he writes a short opinion of the slave trade, showing at once his sound judgement:

In Africa, the poor wretched natives who are blessed with the most fertile and luxurious soil - are rendered so much the more miserable for what Providence meant as a blessing:-the Christian's abominable traffic for slaves - and the horrid cruelty and treachery of the petty kings - encouraged by their Christian customers - who carry them strong liquors - to enflame their national madness - and powder - and bad fire arms - to furnish them with the hellish means of killing and kidnapping. - But enough - it is a subject that sours my blood - and I am sure will not please the friendly bent of your social affections. - I mentioned these only to guard my friend against being too hasty in condemning the knavery of a people who as bad as they may be - possibly - were made worse - by their Christian visitors. - Make human nature they study - wherever thou residest - whatever the religion - or the complexion - study their hearts.

Sancho was very keen on music, and published three collections in his lifetime.  After his death a collection of his letters were published, recording excellent vignettes on the life of a gentleman of middling social status at the time.  In the summer of 1779, he writes of his hopes that the family dog, Nutts, will not catch fleas in the heat, and in September of 1780, he writes to his friend Mrs Cocksedge that he has cast his 'free vote' in the election of that year, in favour of Charles James Fox.  This small note makes Ignatius Sancho the first recorded Black voter in Britain. 

He died in December of that year, the Gentleman's Magazine recording, the first known British obituary of a Black individual, the demise of 'the Extraordinary Negro' Ignatius Sancho, Butler, and Grocer of Westminster.