Fenningham made regular appearances as an attorney in suits of novel disseisin heard at the Sussex assizes held at East Grinstead and Lewes from 1410 onwards, on one occasion receiving a brief from John Woghere. Early on in his career he made the acquaintance of John Halle II, a more prominent member of the legal profession, with whom he was to keep in close touch until Halle’s death. In July 1411 he stood surety at the Exchequer on his colleague’s behalf. He was steadily building up a practice in the central courts. For example, in the Easter term of 1422 he was engaged as an attorney by John Reyson (the bailiff of Brede, Sussex, for Sir John Cornwall and his wife, Henry IV’s sister) to defend him in the court of common pleas in a case of illegal distraint and detinue. During the following Michaelmas term (which coincinded with Fenningham’s second Parliament) he acted in a plea of debt brought by a man from Greenwich, for Cornwall himself in a suit for damages of £40 against a farmer from Horsted Keynes, and on his own behalf in an action against five husbandmen of Seaford and its locality, each of whom allegedly owed him £2.
Fenningham attended the shire court held at Chichester for the Sussex elections to the Parliament of 1429. His reputation in the county was growing; three different boroughs returned him as their parliamentary representative in the 1430s. During the same decade he was also much preoccupied with the affairs of John Halle, both as a trustee of his manorial holdings and as an executor of his will.