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Early career

Benedetto took his first steps into religious life when he was sent to the monastery of the Friars Minor in Velletri, where he was put under the care of his maternal uncle Fra Leonardo Patrasso. He was granted a canonry at the cathedral in the family's stronghold of Anagni, with the permission of Pope Alexander IV. The earliest record of him is as a witness to an act of Bishop Pandulf of Anagni on 16 October 1250. In 1252, when his paternal uncle Pietro Caetani became Bishop of Todi, in Umbria, Benedetto followed him to Todi and began his legal studies there.

His uncle Pietro granted him a canonry in the Cathedral of Todi in 1260. He also came into possession of the small nearby castello of Sismano, a place with twenty-one fires (hearths, families). In later years Father Vitalis, the Prior of S. Egidio de S. Gemino in Narni testified that he knew him and conversed with him in Todi and that Benedetto was in a school run by Rouchetus, a Doctor of Laws, from that city.

Benedetto never forgot his roots in Todi, later describing the city as "the dwelling place of his early youth", the city which "nourished him while still of tender years", and as a place where he "held lasting memories". Later in life he repeatedly expressed his gratitude to Anagni, Todi, and his family.

In 1264 Benedetto entered the Roman Curia, perhaps with the office of Advocatus. He served as secretary to Cardinal Simon de Brion, the future Pope Martin IV, on a mission to France. Cardinal Simon had been appointed by Pope Urban IV (Jacques Pantaléon), between 25 and 27 April 1264, to engage in negotiations with Charles of Anjou, Comte de Provence, over the Crown of Naples and Sicily. On 1 May 1264 he was given permission to appoint two or three tabelliones (secretaries) for his mission, one of whom was Benedetto.

On 26 February 1265, only eleven days after his coronation, the new pope, Pope Clement IV wrote to Cardinal Simon, telling him to break off negotiations and travel immediately to Provence, where he would receive further instructions. On the same day, Clement wrote to Charles of Anjou, informing him that the pope had 35 conditions that Charles must agree to in accepting the crown; he also wrote to Henry III of England and his son Edmund that they had never been possessors of the Kingdom of Sicily. He also commended to the Cardinal the Sienese bankers who had been working for Urban IV to raise funds for Charles of Anjou, and that he should transfer to them some 7,000 pounds Tournois from the decima (ten percent tax) of France. On 20 March 1265, in order to expedite the business with Charles of Anjou, Cardinal Simon was authorized to provide benefices from cathedrals or otherwise within his province to five of his clerics. This may have been the occasion on which Benedetto Caetani acquired at least some of his French benefices. On 9 April 1265, on the petition of Cardinal Simon de Brion, the legation which had been assigned him by Pope Urban was declared not to have expired on the death of Urban IV. There would have been no point in making such a ruling if Cardinal Simon had already ceased to be Legate.

On 4 May 1265 Cardinal Ottobono Fieschi was appointed Apostolic Legate to England, Scotland, Wales, and Ireland by the new Pope Clement IV. In fact, he was sent as the successor of Cardinal Guy Folques, who had been elected Clement IV on 5 February 1265. On 29 August 1265 the Cardinal was received at the French Court by King Louis IX. There he learned that Simon de Montfort and his son Henry had been killed at the Battle of Evesham earlier that month. Cardinal Ottobono did not reach Boulogne until October 1265. He was accompanied by Benedetto Caetani. He was in England until July 1268, working to suppress the remnants of Simon de Montfort's barons who were still in arms against King Henry III of England. To finance their rebellion, the barons had imposed a 10% tax on church property, which the Pope wanted back because the tithe was uncanonical. This drawback was a major concern of Cardinal Ottobono and his entourage. While in England, Benedetto Caetani became rector of St. Lawrence's church in Towcester, Northamptonshire.

Upon Benedetto's return from England, there is an eight-year period in which nothing is known about his life. This period, however, included the long vacancy of the papal throne from 29 November 1268 to February 1272, when Pope Gregory X accepted the papal throne. It also includes the time span when Pope Gregory and his cardinals went to France in 1273 for the second Council of Lyon, as well as the Eighth Crusade, led by Louis IX, in 1270. The Pope and some of the cardinals began their return to Italy at the end of November 1275. Pope Gregory celebrated Christmas in Arezzo and died there on 10 January 1276. In 1276, however, Benedetto was sent to France to supervise the collection of a tithe, which is perhaps when he held the office of Advocatus in the Roman Curia, and then was appointed a papal Notary in the late 1270s. During this time, Benedetto accumulated seventeen benefices, which he was permitted to keep when he was promoted. Some of these are enumerated in a bull by Pope Martin IV, in which he bestowed the deaconry of S. Nicolas in Carcere on Cardinal Benedetto Caetani.

At Orvieto, on 12 April 1281, Pope Martin IV created Benedetto Caetani cardinal deacon of Saint Nicholas in Carcere. In 1288 he was sent as Legate to Umbria to attempt to calm the strife between Guelphs and Ghibellines, which was taking the form of a war between the cities of Perugia and Foligno. In the winter of 1289, he was one of Pope Nicholas IV's advisors as he decided on a settlement of the disputes over the election or appointment of Portuguese bishops, in which King Denis played a major role. To give greater authority to the final mandate of the Pope, Cardinal Latino Orsini of Ostia, Cardinal Pietro Peregrosso of S. Marco, and Cardinal Benedetto of S. Nicola in Carcere appended their signatures and seals. Three years later, on 22 September 1291, Pope Nicholas IV (Girolamo Maschi d'Ascoli, O.Min.) promoted him to the Order of Cardinal Priests, with the title of SS. Silvester and Martin. Given the fact that there were only a dozen cardinals, Cardinal Benedetto was assigned the care (commenda) of the deaconry of S. Agata, and his old deaconry of S. Nicola in Carcere. As cardinal, he served as papal legate in diplomatic negotiations to France, Naples, Sicily, and Aragon.