As the film of a woman's amazing life is released, a new biography of Lord Mansfield looks at his role in abolishing the slave trade

She wore the finest silks, lived in one of London's most desirable homes and studied in a library still regarded as one of the greatest achievements of the renowned 18th-century designer John Adam. Yet Dido Belle was the daughter of an unknown black slave woman so could not sit at the dinner table with her adopted family at Kenwood House in north London.

Belle, a film directed by Amma Asante and released in America this weekend, tells the story of the illegitimate young woman who found herself among the household of Lord Mansfield, one of the greatest men of the Georgian age. As lord chief justice, in 1772 he ruled that a master could not take a slave out of Britain by force, a judgment seen as a key stage in the eventual abolition of the slave trade.

However a new biography of Mansfield, Dido's great-uncle and benefactor, has revealed that this complex figure was not the crusading liberal portrayed in the film. "He was a brilliant mind, but was chiefly interested in protecting the status quo," said author Norman Poser.

In Lord Mansfield, Justice in the Age of Reason, Poser, a New York academic, argues that the eminent politician and judge does not merit his reputation as a radical hero of the anti-slavery movement. Although he was fond of Dido Belle and always fair to her, there is no evidence that she influenced his views. In fact, the judge's famous ruling in the case that later helped to dismantle slavery was the result of his slow acknowledgement that slaves had rights in law.

"The film is right to suggest Mansfield is closely associated with the cause, but he did not want to bring down the slave owners in America, or even to end slavery," Poser said.

"He did, however, eventually rule that the practice was so suspect it would have to be set down in deliberate legislation if it was to continue, rather than simply being supported by common law and precedent."

Mansfield's ruling on slavery in the "Somerset case" of 1772 reads: "It is so odious that nothing can be suffered to support it, but positive law." Invoking habeas corpus, he reasoned that the captain of the ship on which the slave was being deported could fairly be considered to be holding a man against his will.

"He was very reluctant to annoy the slave owners and vested interests," said Poser. "He rather hoped things would just go on as they were, saying 'I would have all masters think they were free and all negroes think they were not'."

Poser, 86, who has retired from teaching law, became intrigued by the story of Mansfield's home life when he saw the portrait of Dido Belle and her cousin during a visit to Scone Palace, the Mansfield family's Scottish home, in 2006: "I was also drawn to his remarkably clear legal decisions. He did not just give a ruling; he gave guidance on principles of law, many of which are still cited in cases on both sides of the Atlantic."

Belle, which stars Gugu Mbatha-Raw in the title role, comes out in Britain on 13 June and has already been compared to Steve McQueen's Oscar-winning film 12 Years a Slave. Poser welcomes the British film and recognises that Misan Sagay's screenplay needed to flesh out the scant details of Dido Belle's story, yet he believes Mansfield was a more complex and powerful figure than actor Tom Wilkinson has the chance to convey.

The second son of a "questionable" Scottish Jacobite family, William Murray, as he was born, made a great deal of money as a young barrister before going on to serve as solicitor general, attorney general, Chancellor of the Exchequer, Speaker and finally lord chief justice.

At the centre of London society, he was friends with John Adam, poet Alexander Pope, actor David Garrick, painter Joshua Reynolds and Samuel Johnson's biographer, James Boswell, who regarded him as a man of "cold reserve and sharpness" who "chills the most generous blood".

In Asante's film Belle finds crucial evidence for her uncle while he presides over a case involving the mass drowning of slaves in the Caribbean. The real Dido Belle came to Kenwood because her father, a sea captain, brought her to his uncle as a companion to her cousin Elizabeth, the young woman she is pictured with in the Scone portrait.

Dido is thought to have joined Lady Mansfield in running a small dairy in the grounds of Kenwood, but Asante did not film in London, where Kenwood was undergoing restoration, but at Syon House and Osterley Park House in Middlesex, and in Buckinghamshire at West Wycombe House. (Cinemagoers in search of the real Kenwood can spot it in Richard Curtis' Notting Hill).

After Lady Mansfield's death, Dido stayed on to care for her uncle, reading newspapers to him at breakfast as he became crippled with rheumatism. He died at 88 in 1793, leaving Dido a provision in his will as well as setting aside a substantial one-off payment in a codicil.

He wrote: "I think it right considering how she has been bred and how she has behaved to make a better provision for Dido." After his death, she married and had three sons, dying in her early forties in 1800.

Poser's book tells the story of a generous and clever man, who read the mood of his times rather than setting out to abolish slavery. "A distinction should be made between Mansfield's decisions as a judge who consistently furthered the interests of merchants and property owners, even at the expense of fundamental human rights, and his humane conduct of his personal life," he writes.

But he argues that Mansfield's wider influence lives on in every branch of the law, from contract law to freedom of the press, where he ruled against prior censorship. "My daughter, who teaches law in Nebraska, told me he had been cited in the ruling of a recent paternity case there. It is strange that his name is not better remembered in Britain and America."