Maria was one of the three the illegitimate daughters of Hon. Edward Walpole of Frogmore House in Windsor, by Dorothy Clements, a charming and beautiful milliner from Darlington in Co. Durham. This lady bore him five children but, although a faithful lover, he never married her because he feared the opposition of his father, Sir Robert. The surviving children, three daughters, were, however, brought up as Walpoles and recognised as such by the family. Edward had his own friends and Windsor people "of any fashion," unfortunately, ignored these young ladies, until eventually Mrs. Ewer, the wife of one of the canons at St. George's Chapel, befriended them. Another canon, Frederick Keppel, subsequently Bishop of Exeter and Dean of St. George's, married Laura, the elder of the sisters, in 1758 and his mother, Lady Albemarle, introduced both Maria and her younger sister, Charlotte, at Court. Maria, then twenty-two, was the queen of the bunch. Her uncle, Horace Walpole, called her "Beauty itself" and, when he introduced her to the forty-three-year-old Lord Waldegrave, the latter was soon captivated, despite his being generally thought of as a confirmed bachelor. Maria's extravagant tastes encouraged her not to be put off by his apparent lack of personal hygiene and Dr. Keppel tied the knot at Luxborough House, Sir Edward's Pall Mall home, on 15th May 1759. After a family dinner-party, the pair left for Navestock at 8pm, the bride wearing "a white and silver nightgown and a hat very much pulled over her face." A year later, Charlotte, followed her sister in a noble marriage, becoming the wife of Lord Huntingtower. As their Uncle Horace put it, "Don't we manage well?"

Lady Mary Wortley Montagu wrote from Padua in June 1759, "I am not surprised at Lady Waldegrave's good fortune. Beauty has a large prerogative." Lady Coventry, the more famous Miss Gunning and the new Countess's only possible rival, made the best of things by striking up a great alliance with her, and the pair were thence constantly to be seen together. Horace Walpole tells of their being mobbed in the Park and, at one time, described his home of Strawberry Hill as "a perfect Paphos, a land of beauties" since, not only the peerless pair, but the Duchess of Richmond, Lady Ailesbury and the Duchess of Hamilton (the other Miss Gunning) had all been dining there. "A thousand years hence," he wrote, "when I begin to grow old, if that can ever be, I shall tell of that event, and tell young people how much handsomer the women of my time were than they will be then." He was not above criticising his nieces, however, and admitted that their chit-chat sometimes bored him: "not that the trifles are less important than those of one's own time, but the preceding age always appears respectable to us (I mean as one advances in years), one's own age interesting, the coming age neither one nor t'other."

Maria's husband, James, Earl Waldegrave, was an intimate friend and adviser of King George II. A man of great character, he preferred to exercise his influence from behind the scenes without playing any open part in public affairs - the 'power behind the throne'. Much against his will, he became governor to the future George III in 1752, but resigned "this most painful servitude" four years later, acting as premier for a few days shortly afterwards and being rewarded with the Order of the Garter. Circumstances gradually forced him to a more and more obvious share in active politics, and he must have succeeded to the leadership of the Whigs at the crisis that occurred soon after his death from smallpox in 1763.

The widowed Maria and her three infant daughters were not overly well provided for, as, though she was made sole executrix and left everything the Earl had, most of his income came from his lucrative appointments and therefore died with him. Her Uncle Horace rented Ragman's Castle at Twickenham (Middlesex) for them, but was too acutely aware of his niece's attractiveness to hope to have her near him for any length of time.

Adoring Eton boys were soon crowding to St. George's to gaze on "Lady Waldegrave at Castle prayers" and, before long, she astonished society by refusing the hand of the Duke of Portland and becoming attached to George III's favourite brother, William, a Prince of good reputation, eight years her junior. His obvious devotion, people could understand; hers, equally patent, they could not, especially as the Duke of Gloucester had no great means and there seemed little prospect of the King's sanction to the match. The pair were constantly together and gossip began to say that, if they were not married, they ought to be. The Duke rented the Manor Lodge in Old Windsor and contemplated buying the lease but, in 1767, became Warden of Windsor Forest with Cranbourne Lodge at his disposal. He made great improvements there and Lady Waldegrave commissioned Thomas Sandby to extend 'Forest Lodge' on the top of St. Leonard's Hill nearby, a property called, by a 1771 Guide Book, "a noble edifice commanding a most extensive and delightful prospect of the Thames". She also leased a house in Portman Square (Westminster) quite out of proportion to her own income of £1,000 a year. Having written written his Historic Doubts on Richard III, Horace Walpole's friends suggested a companion volume about a second Duke of Gloucester. Horace was as much in the dark as the rest of the World and, when he summoned up courage to question his niece, met with a rebuff which prevented him opening his mouth on the subject again.

Henry Frederick, Duke of Cumberland, another brother of George III, had been appointed Ranger of Windsor Great Park in 1766. Towards the end of 1771, he horrified the Court by suddenly announcing his marriage to a Mrs. Horton, according to the usual authority, "a young widow of twenty-four, extremely pretty, not handsome, very well made, with the most amorous eyes in the World, and eye-lashes a yard long; coquette beyond measure, artful as Cleopatra, and completely mistress of all her passions and projects." The new lady of Cumberland Lodge was "a well known leader of riotous parties at Vauxhall and Ranelagh" and Lady Ann Fordyce remarked that "after hearing her talk one ought to go home and wash one's ears." George III promptly banished her husband from Court and, determined to uphold the dignity of the Crown, had the Royal Marriage Act passed in March 1772.

This legislation, Lady Waldegrave's condition and a certain amount of sympathy with his younger brother soon forced Gloucester to make a similar announcement. The Duchess of Cumberland had taken every precaution and could produce registers, certificates and witnesses to prove her case. The Duchess of Gloucester's position was very different: she had been married at her own house in Pall Mall on 6th September 1766, without any witnesses, and by her own chaplain, who died shortly afterwards. So the evidence for the union rested on the sworn statements of husband and wife, at that time perfectly adequate. The authenticity of the ceremony was not questioned and Parliament pronounced in its favour, with the result that the girl at one time entitled to no name but 'Maria' could write herself 'Princess of Great Britain and Ireland'. Though she was never received at Court.

Maria had no ambition for any such rank and would much rather have been called plain 'Lady Waldegrave'. She was in love with her husband, they were married and nothing else mattered. From his own point of view, it would have been wiser if the Duke had made the facts public earlier. The circumstances left George III no option but to treat the two brothers alike and his Grace of Gloucester felt banishment from Court sorely. He employed himself by improving St. Leonard's Hill - thence called Gloucester Lodge - and had Sandby landscape the grounds and add plantations. He also bought the old house lower down the hill called the 'Hermitage', renaming it 'Sophia Farm' (now 'St. Leonard's') after his daughter who was born on 29th May 1773. A second girl died as the result of inoculation for smallpox and a son arrived while the pair were on a debt-imposed exile abroad, with which the King had little sympathy. George III and his brother became reconciled, however, in 1778 when little Prince William and Princess Sophia were granted £8,000 and £4,000 respectively. They sold their twin St. Leonard's estates in 1781 for the princely sum of £10,000 and separated six years later after a dispute over their daughter's education.

The Duchess of Gloucester died, greatly respected, in 1807. Her three daughters, by her first marriage, had received lodgings at Windsor Castle in 1772, and all married well; the other daughter, Sophia, "a jolly frank Princess, full of good fun and good sense" who lived at the New Lodge in Winkfield, remained single. Dorothy Clements' grandson, the second Duke of Gloucester, was kept in reserve as a possible Prince Consort of England, should a suitable foreign husband not be found for Princess Charlotte of Wales. A few weeks after her wedding, in 1816, he married his old flame, Princess Mary, George III's daughter and Queen Victoria's favourite aunt. All the Gloucesters were buried in St. George's Chapel at Windsor Castle.