Knollys was sent to university with his elder brother Robert, but thereafter their paths diverged. While Robert embarked on a military career and foreign travel and remained unattached, Knollys married the daughter of a Buckinghamshire squire and settled in his native Reading. The date on which his knighthood was bestowed has not been firmly established, but it was sometime before 5 Mar. 1623, when the Reading corporation referred in its minutes to his father as Sir Francis ‘the elder’. During the 1620s Knollys was elected to Parliament for Reading on four successive occasions, each time backed by his uncle, Viscount Wallingford (William Knollys†). In the contested elections of 1626 and 1628 John Saunders* garnered more votes than Knollys, but in deference to the latter’s superior social status Saunders was always returned as the junior burgess.8 Reading Recs. ii. 169, 270, 273, 386-7; HMC 11th Rep. VII, p. 221. Knollys played almost no recorded part in the parliaments of the 1620s. However, on 3 Feb. 1629 he was added to the committee that was appointed to examine the information that was to be presented that afternoon by the merchants John Rolle* and Richard Chambers concerning the seizure of their goods by the customs officers.9 CJ, i. 926a.
The elder Sir Francis Knollys proved extraordinarily long-lived, and consequently Knollys himself was denied local office until his father was in his dotage: the claim that he was a magistrate as early as 1627 is questionable.10 M.F. Keeler, Long Parl. 244, bases her assertion on Reading Recs. ii. 353, which shows merely that he was present with other leading figures in the town at the reading of the will of the wealthy clothier John Kendrick. Both men were returned to Parliament for Reading in April 1640, and were subsequently elected to the Long Parliament later that year. On both occasions Knollys took the junior seat. He expired in May 1643, outlived by his father, and was buried in the church of St. Laurence, Reading. No will or administration has been found.