She was married, by her cousin’s husband, King Henry VII, to Sir Richard Pole, son of Sir Geoffrey Pole of Ellenborough and Medmenham in Buckinghamshire, and his wife, Edith St. John, who was half-sister of the King's mother, Margaret Beaufort. Sir Richard was the widower of Alice, the daughter of Edward Langford of Bradfield Manor in Berkshire and widow of Sir John Stradling of Dauntesy in Wiltshire. This poor lady had died in 1488, having lived to see her son, Sir Edward Stradling, reach his late teens, only to be murdered with the rest of his household during an armed robbery at Dauntesy Manor the year before. Traditionally, this was said to be no random attack, but an organised assault arranged by Sir Edward's brother-in-law, Sir John Danvers of Calthorpe House in Oxfordshire (half-nephew of Sir Richard's step-mother as well as her brother, Justice Sir William Danvers of Chamberhouse Castle in Crookham, Berkshire). Sir Richard was eleven years Princess Margaret’s senior, and King Henry made him a squire of his bodyguard and Knight of the Garter. He also gave him various offices in Wales, making him Constable of Harlech and Montgomery Castles and Sheriff of Merioneth. He held, too, the Controllership of the Port of Bristol. Margaret’s marriage to Richard took place on 22nd September 1494. The following year, Pole seems to have raised men against the pretender to the Throne, Perkin Warbeck. In 1497, he was retained to serve against the Scots, with five demi-lancers and 200 archers, and, shortly afterwards, with 600 men-at-arms, 60 demi­-lancers and 540 bowmen and billmen. Two or three years later, he was appointed Chief Gentleman of the Bedchamber to Prince Arthur, whom he attended at Ludlow Castle and into Wales after his marriage to Catherine of Aragon, and the chief government of the marches was committed to his charge. He died in December 1505, leaving his widow with a family of five children: Henry (who became Baron Montague), Arthur (buried at Bisham Priory), Regi­nald the cardinal, Sir Geoffrey (who married one of the Pakenham sisters from East Court, Finchampstead and Lordington House in West Sussex; their daughter, Mary, became the ancestress of the Cuffauds of Cuffaud Manor in Old Basing) and their only daughter, Ursula, who married Henry, Baron Stafford, son of the Duke of Buckingham, on 16th February 1519.

Henry VII was worried about Margaret's brother Edward, Earl of War­wick’s claim to his throne. So, in 1499, he had him executed on a trumped up charge of trying to escape from custody in the Tower with Perkin Warbeck. He was buried at Bisham Priory. This monarch's son, Henry VIII, who described Margaret as the “most saintly woman in England,” was anxious, after his accession, to atone for this injustice. He, there­fore, granted Margaret an annuity of £100, on 4th August 1509, and, on 14th October 1513, he created her Countess of Salisbury, and gave her the family lands of her mother’s inheritance, the Earldom of Salisbury, chief of which was Bisham ‘Abbey’ Manor. Her brother's attainder was reversed in the parliament of 1513/14 and full restitution was made to her of the rights of her family. She thus became possessed of a very magnificent collection of property, lying chiefly in Hampshire, Wiltshire, the western counties and Essex. However, there is little doubt that this was heavily burdened by redemption-money claimed by the King. On 25th May 1512, she had delivered to the King’s chief minister, Cardinal Wolsey, £1,000 as the first payment of a benevolence of five thousand marks for the King's wars, and, in 1528, she was sued for a further instalment of £2,333-6s-8d. Of her restored lands, the manor of Canford in Dorset and some others were soon reclaimed by the Crown as part of the Earldom of Somerset. In 1532, she purchased the manor Aston Clinton in Buckinghamshire from Sir John Gage, but later sold it to her steward, John Babham of Cookham. She made Warblington Castle, on the Hampshire Coast, her chief residence, but was also often at Bisham which was within easy reach of both London and Windsor. The existing dovecote there was built for her use. King Henry and Queen Catherine are known to have visited for extended periods on numerous occasions and, 1518, the two-year-old Princess Mary was called to join them. The King seems to have much enjoyed the hunting there.

A few years later, Margaret was made the Princess's governess. In 1521, at the time of the Duke of Buckingham's attainder, she and her sons seem to have been under a momen­tary cloud, but the Countess herself was allowed, however, to remain at court “because of her nobility and goodness”. In 1525, she went, with Princess Mary, to Wales. In the summer of 1526, during her absence, the King visited her castle at Warblington.

In 1533, when the King married Anne Boleyn, Margaret’s loyalty was severely tried. She refused to give up Princess Mary's jewels to a lady sent from court and was discharged from her position as governess. She declared that she would still follow and serve the princess at her own expense. Her self-sacrificing fidelity to the princess was fully recognised by Catherine of Aragon. The King, however, took good care to separate his daughter from one whom she regarded as a second mother. The Countess seems to have retired to the country. She was certainly at Bisham 'Abbey' Manor when the King's commissioners, Richard Layton and Edward Carne, arrived to demand the closure of the adjoining priory. For Layton complained of having been threatened by 'My Lady Salisbury' and her household. She also impressed upon the prior never to surrender.

After Anne Boleyn's fall in 1536, the Countess returned to Court and it may have been as something of a favour to her that King Henry chose to re-found Bisham Priory as an abbey, although this did not come about until the lady had, again, fallen from favour. For it was at this very time that her son, Reginald, sent, to the King, his book, ‘For the Defence of Church Unity' which denounced Henry's anti-Catholic religious policies and, naturally, gave deep offence. So the Countess received the monarch’s extreme displeasure as a result. Both she and her eldest son, Lord Montague, wrote to Reginald in strong language of reproof. She denounced him as a traitor to her own servants and even expressed her grief that she had given birth to him. The letters, however, were written to be shown to the King's Council, by whom they were despatched to Reginald in Italy. Though the Countess's alarm was quite genuine, her disapproval of Reginald's proceedings was not equally sincere. The King knew well that his policy was disliked by the whole family and he privately told the French Ambassador that he intended to destroy them all. The blow fell in the autumn of 1538, when her sons Geoffrey and Henry, Lord Montague were arrested. One Gervase Tyndall, a spy upon the Countess's household, was called before the King’s new chief minister, Thomas Cromwell, at Lewes in East Sussex. He reported a number of circumstances about the escape, some years before, of the Countess's chaplain, John Helyar, Rector of Warblington, beyond the sea and about clandestine messages sent abroad to Cardinal Pole himself, via one Hugh Holland, who Babham had already tried to curb. FitzWilliam, Earl of Southampton, and Goodrich, Bishop of Ely, were sent down to Warblington to examine the Countess. They questioned her all day, from morning till almost night, but could not wring any admission from her. They nevertheless seized her goods and car­ried her off to FitzWilliam's house, Cowdray at Midhurst in West Sussex. Her castle at Warblington was thoroughly searched and some letters and Papal bulls dis­covered. Her persecutors renewed the attack with a set of written interrogations and ob­tained her signature to the answers. She re­mained in FitzWilliam's house, long unvisited either by him or his countess, until 14th March 1539, when, in answer to her com­plaints, he saw her and addressed her with barbarous incivility. Shortly afterwards, she was removed to the Tower of London. In the May, a sweep­ing act of attainder was passed by Parlia­ment against, not only Lord Montague and his cousin, the Marquis of Exeter, who had already suffered death, but against the Countess. However, she was not even called to an­swer the accusations against her, and against her son, Reginald, and many others. At the third reading of the bill in the House of Lords, Cromwell produced, what was taken as evi­dence of treason, a tunic of white silk, which, it was said, FitzWilliam had found in her house. It was em­broidered with the arms of England, surrounded by a wreath of pansies and marigolds, having, on the back, the badge of the five wounds of Christ carried by the Northern rebels during the Pilgrimage of Grace. The act of parliament was passed on 12th May 1539, but it was not put into force at once. In fact, by April 1540, it was supposed that the Countess would be released. She was tor­mented in prison by the severity of the wea­ther and the insufficiency of her clothing. In April 1541, there was another insurrection in Yorkshire under Sir John Neville and, apparently on this account, it was resolved to put the Countess to death, without any further process but under the act of attainder passed two years before.

Early in the morning of 27th May, the Countess of Salisbury was told that she was to die. She replied that she had not been accused of any crime, but reports of what happened next vary. She may have walked boldly from her cell to East Smithfield Green, which was within the pre­cincts of the Tower, or she may have been dragged kicking and screaming despite her sixty-seven years. No scaffold was erected, but there was only a low block. The Lord Mayor of London and a select company were present to witness the execution. The Countess may have com­mended her soul to God and asked the by­standers to pray for the King and Queen, Prince Edward and the Princess Mary, her god-daughter, to whom she desired to be specially commended. She then, as com­manded, laid her head upon the block; or her head may have been forced down by her gaolers. All agree that the exe­cutioner was a clumsy novice, who hideously hacked her neck and shoulders before the decapitation was accomplished after some eleven blows. One story, probably invented to explain these numerous blows, claims that she leapt up after the first missed and was chased around by the axeman swinging his axe. Apparently, on the walls of her cell was, afterwards, discovered this inscription:

For traitors on the block should die;
I am no traitor, no, not I!
My faithfulness stands fast and so,
Towards the block I shall not go!
Nor make one step, as you shall see;
Christ in Thy Mercy, save Thou me!