Emma Harcourt was the daughter of Richard the Seigneur de Harcourt. They were cousins of the Harcourts of Stanton Harcourt in Oxfordshire. She married firstly, Sir John St. John of Stanton St. John in Oxfordshire and Swallowfield in Berkshire. He had been married previously to a daughter of his guardian, Geoffrey dc Lucy.

John de St. John accompanied King Richard I to the Holy Land and was at the Siege of Acre. He was one of the knights whom the King "on the inspiration of St. George, had distinguished by tying a leathern thong or garter round the left leg to incite the wearer to greater daring" and this is one of the legends cited as the first institution of the Order of the Garter. He died in January 1230 and was buried in Oseney Abbey in Oxford.

Emma continued to hold Stanton and Swallowfield in dower; and, by the end of the same year, she had remarried to Geoffrey Le Despencer, from Martley in Worcestershire. He subsequently paid £100 for the wardship of her son, Roger St. John. Geoffrey Le Despencer was grand-uncle to Hugh Le Despencer, first Baron Le Despencer. He died in 1252, leaving a son, John Le Despencer, who succeeded to the possession of the "Castle of Swallowfield" as it is called in a roll of this date. In the following July, Emma, gave 400 marks (£266-13s-4d) for the custody of her son, John, and his lands. Her eldest son, Roger St. John (eventually the first and last Baron St. John of Stanton), seems to have resided at Stanton St. John.

In 1253, Emma Despencer was appointed Lady in charge of Princess Katherine, the youngest daughter of King Henry III, who was born on 25th November of that year. At the Feast of the Circumcision, we find the Queen presented her with a brooch and, later on, with a girdle to the value of 21s-2d; and to Dionisia, Damsel of Emma Despencer, also a brooch.

The little Princess was deaf and dumb, but of great beauty and idolized by her Royal parents. She was christened with much pomp by Boniface, Archbishop of Canterbury, the Queen's uncle, who stood as godfather. The infant princess received the name of Katherine because she had been born on that saint's feast day. The King held a great banquet in honour of the christening on St. Edward's Day (5th January) 1254, to which he invited all the nobility, including "Emma de St. John of Swalefeld and her son." Amongst the provisions on this occasion were "fourteen wild boars, twenty-four swans, one hundred and thirty-five rabbits, two hundred and fifty partridges, fifty hares, two hundred and fifty wild duck, sixteen hundred and fifty fowls, thirty-six female geese and sixty-one thousand eggs."

Soon after this, Queen Eleanor had to join the King in Gascony and left her infant at Windsor, under the charge of Emma Despencer and two nurses, Avisa and Agnes. Early in the next year, the King and Queen returned to England and, by an order dated from Merton 2nd April, gold clothes, with borders embroidered with the King's coat-of-arms, were to be made for the King to offer in Westminster Abbey for his daughter Katharine. In the Autumn, the little princess became ill and she was sent to Swallowfield under the care of Emma Despenser. For her amusement a young goat was brought there from the King's forest at Windsor. The change seemed to benefit her for a time but, in the Spring of 1256, she had a relapse. By the King's command, a report of her condition was sent to him by special messenger during his expedition to France and, when he heard of her convalescence he ordered that a "silver image made after the likeness of a woman" should be placed in Westminster Abbey as a votive offering, and the bearer of the news was given "a good robe."

We also find orders for her expenses and for those of several children who were companions to the little Princess. Notwithstanding, however, all the care bestowed upon her, the little Katherine died in 1258, aged five years, to the great grief of her parents, the Queen becoming seriously ill after her death. The King presented the nurses with a present equal to £100 of our money. There was a magnificent funeral, which cost £51-12s-4d. The Princess was buried in the ambulatory in Westminster Abbey, in the space between the chapels of King Edward and St. Benet, close to the tomb of her uncle William de Valence. A splendid monument was raised to her memory by the King, rich with serpentine and mosaics, and surmounted by a silver image of his child as St. Katharine, made by the King's goldsmith at the cost of 70 marks (£46-13s-4d). The Hermit of Charing was paid fifty shillings a year as long as he lived, that he might support a chaplain to pray daily at the Chapel of the Hermitage for the soul of Princess Katherine.

Emma's son, John Le Despencer came of age in 1256. No doubt he and his wife then lived at Swallowfield for we find, in a Close Roll of 1256, that permission was given for Emma, Lady of Swallowfield, to dwell in Portchester Castle (Hampshire) and an order was issued for William Turberville, Warden of the Castle, to answer to the Exchequer for the issues thereof. She seems to have died in late 1270 and was buried alongside her first husband in Oseney Abbey.