The son of a Lancastrian retainer, Forster was himself in the service of John of Gaunt by 1390, and before the duke’s death received from him an annuity of four marks. It is not surprising, therefore, to find his fortunes much advanced by the accession of Henry IV. Soon after that event, on 13 Nov. 1399, he shared with another ‘King’s servant’ a grant of £30 owed to the Crown as a fine, and at the end of the same month he was granted a duchy of Lancaster annuity of eight marks for life. By October of the next year he was discharging the office of serjeant of the royal buttery, receiving as such a third annuity, of ten marks, paid this time by the Exchequer, although he relinquished this particular fee in 1402. How long he retained the serjeanty is unknown, but he possibly gave it up on appointment to the office of butler of Calais, a post worth 20 marks a year which he was occupying by April 1401.
Forster’s services to the Crown were rewarded in August 1401 by a grant to him and his heirs of the duchy of Lancaster manor of Hessle, near Wakefield: he took possession in February 1403, and thereafter occupied the manor until his death. Little is known of his activities during the next five years, perhaps because of absence in Calais, but he was still employed in the Household in April 1408 when, by this time yeoman of the King’s private cellar, he was granted the office of bailiff of Sandwich, with an annual fee of £20. In December of the same year he obtained a forfeited tenement in Sandwich, and in November 1409 he was confirmed in the bailiffship for life, at an increased fee of £24. He exercised this office by deputy, simultaneously with an increasing number of other posts granted to him in the closing years of Henry IV’s reign. In 1408 he became duchy feodary in three of the Midland counties, which position he maintained for two years. By November 1411 he was serjeant of the royal pantry. A year later he received a share in the goods, worth £40, of three felons, and by January 1413 he was a Kings esquire: in the same month he was granted the office of bailiff of Leicester, to exercise personally or by deputy, ‘all other grants notwithstanding’.
The end of Henry IV’s reign appears to have marked a turning point in Forster’s career, for, though he was confirmed as bailiff of Leicester by the new King, he soon gave up his far more lucrative office at Sandwich, and was apparently never again employed in the royal household.
Forster witnessed many local deeds, and in 1418 acquired some 24 messuages, 17 shops and another property in the town, the rent from which he made over to the Newarke college and hospital, which were Lancastrian foundations. In 1422 he witnessed the borough’s parliamentary election, and in the following year was elected mayor, having given up his bailiff’s office as a result of the Council’s decision to lease the bailiwick to the corporation.