Little is known of his early years, but he was a royal official in Cumberland and Westmorland for all his adult life, serving as member of parliament, escheator and justice of the peace on multiple occasions.

Although never wealthy, de la More was a man of social and political significance in the area and regularly acted on behalf of his fellow gentry.

The earlier history of the family is obscure, although they are known to have settled in the county Cumberland, in the north of England, during the reign of King Henry III (1216–1272),[5] and may have held the manor of Kirkdale in 1280.

[10] In 1415, he was discovered leading secret raiding parties across the Scottish border and was arrested by officers of the Warden of the East March for breaking the truce.

[4] At some point, de la More came into lands in Cumcatch, which he augmented by purchasing property in Branthwaite and taking out a 20-year lease on the manors of West Farlam and Sebergham, suburbs of Carlisle, from the Crown.

[3][note 2] Either way, this union would have made de la More brother-in-law to Robert Sandford of Askham, Westmorland, MP for Appleby in 1413, and an important member of the local gentry.

[21] Later indictment records from the local King's Bench show that by 1429, de la More had sheltered one Robert Bell, a yeoman of Patterdale, from the law.

By then it was customary for the Crown to look to this class to fill local administrative and judicial posts, the most important of these being sheriff and justice of the peace.

[3] This was an important position, as elections often took place under pressure from both the Crown and local nobility,[25] and the gentry were a highly political class.

The historian K. B. McFarlane comments that "they were willing to be guided by those who had claims on their support; but it was foolish to attempt to drive them with too tight a rein",[26] and neither Crown nor local aristocracy could guarantee their preferred candidate's election.

[29] That Moresby could commit this political chicanery was due to the original writs of summons being superseded by another that arrived too late for the election.

[31] This thesis, however, has been challenged by Simon Payling, who argues that Leigh lacked full Percy backing, and that in any case, that there is no independent evidence for the existence of such rivalry between the two families at this point.

[33] This indicates, argues the prosopographer Gilbert Bogner, that de la More was a propertied man, effectively at knightly level, and a "prominent" member of the gentry.

For example, he was executor to his colleague Sir Robert Lowther's will in 1430,[36] and sat on an assize assessing William Stapleton's claim to an estate in Black Hall.

[46] It was not his wealth, though, suggests Payling, that led to his important position in Cumberland society: "he owed that to his place in the service of Richard Neville, Earl of Salisbury".

In what appears to have been a political compromise, de la More's co-candidate, Thomas Crackenthorpe, had been in the service of the Earl of Northumberland for several years.

As a close connection of Salisbury's, "it would be surprising if de la More did not play a significant part in thwarting Percy interests"[2] on that occasion, argues Payling.

That there was felt to be some irregularity on de la More's part is clear; the writs had arrived in time only to hold the election a week after parliament had already assembled.

[53] De la More was certainly in the Earl of Salisbury's service by 1452,[55] and had probably been so since 1443,[10] when he was recorded as having witnessed an arbitration between Thomas, Lord Clifford and William Stapleton at Middleham Castle.

During More's shrievalty, the Carlisle Justices were unable to hold their sessions in 1452, and that same year, John Skelton was captured for ransom by the Scots.

[57] By 1454, Salisbury's sons—John and Thomas—were engaged in a bitter feud with the Earl of Northumberland's sons, Thomas Percy, Lord Egremont and Sir Richard.

[3][51][71] This bille is gr[a]nted as it is desired so that he accompt duely by his othe and that it be not taken for a consequent hereaft Due to the families both owning land in the region, the Percy–Neville feud spilt over from Yorkshire into North West England.

[60][note 12] In July 1454, he petitioned the King, by way of Salisbury in chancery, that during de la More's last period as sheriff, Lord Egremont had prevented him from carrying out his official duties.

[74] Egremont, with "certein riotous Peple",[75] de la More complained, had assaulted him, his deputies and his servants, and had threatened to have him beheaded.

[76][62] Egremont's violence, continued de la More, meant that "the one half of the shire was divided from the other" and that his tenure as sheriff was punctuated with "great dissensions, riots and debates".

[60][note 13] Conversely, argues the historian Mark Ormrod, he may have believed the King would be more likely to act on the petition if it came to him from Henry's own chancellor rather than an ex-sheriff.

[78] In context, argues Booth, the incident was part of the wider political struggle for dominance in the West March between the Percy and Neville families.

[4] Meanwhile, factionalism in local Cumberland politics was on the increase following the anti-Yorkist Parliament of Devils in 1459, and both Salisbury, his son Thomas and de la More lost their places on the commission of the peace.

At the outbreak of the Wars of the Roses, Egremont remained loyal to Henry VI and, acting as the King's personal bodyguard, was slain at the Battle of Northampton in June 1460.

[93] This did not prevent her marriage to William Vaux of Catterlen[102] (died c. 1481),[103] whose cousin Roland Vaux—another county MP and sheriff—was also a Neville man and a colleague of de la More's.