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.
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.