Before the general election of 1754 Jennings applied to the Pelhams to bring him into Parliament; they tried to place him but failed.
He appears in Fox’s list of Members favourable to the peace preliminaries, early December 1762, but according to Newcastle’s list voted against them. He voted against the Grenville Administration on Wilkes and general warrants; belonged to Wildmant’s Club; and was classed by Newcastle, 10 May 1764 as a ‘sure friend’. When the Rockingham Administration was being formed Jennings wrote to Newcastle, 13 July 1765:
The many assurances I have had of your Grace’s goodwill towards me encourages me to beg the favour of your recommendation to Lord Rockingham for some employment ... I waited on his Lordship this morning and mentioned my wishes to be at the Board of Trade ... My wish for being at one of the Boards proceeds from my liking business.
Newcastle included Jennings in a list, 15 July 1765, of persons to be given office, and applied to Rockingham on his behalf. On 22 Aug., when most of the vacant places had been filled, Jennings wrote again to Newcastle: ‘I by no means waive my claim, and hope that in case of a vacancy at the Board of Green Cloth or elsewhere I shall still be honoured with your Grace’s protection.’ Rockingham, in his list of July 1765 classed Jennings as ‘pro’, but seems to have made no effort to provide for him. ‘I ... am truly concerned’, wrote Newcastle to Jennings, 17 June 1766, ‘that I have hitherto been able to be of such little service to you.’ Without doubt he was sincere.
After Chatham had taken office Newcastle wrote from Claremont to Rockingham, 31 Aug. 1766:
