He was guided for political life by the rector of Upminster, John Rose Holden, and his Cambridge contemporary, Henry Bickersteth, 1st Baron Langdale, who said, of Barrett-Lennard's future, "it is a sort of treason to yourself and your country when you neglect the opportunities fortunes has bestowed on you".
Although Thomas Dawson, 1st Viscount Cremorne offered his influence, following canvassing Barrett-Lennard declined to go to a poll, paid his respects at Dublin Castle and returned to Belhus, Essex and Scotland.
[3] He remained active politically, and in line with his family's support for the Whigs and Catholic emancipation, he campaigned for a county meeting in Essex to protest the Liverpool ministry's responses to the Peterloo massacre.
At the election's hustings, he denounced ministerial corruption, high taxes, sinecures and repressive legislation passed in the wake of Peterloo, as well as called for free trade and parliamentary reform, arguing Ipswich was in danger of becoming a government borough.
[1][5] He became a regular attender and ready debate, voting against the Tory government and with the Whigs in almost every major division between 1820 and 1824, including supporting the 'Mountain' and Joseph Hume's campaigns for economy and retrenchment and becoming one of their spokesman on diplomatic expenditure, local courts, and a variety of issues affecting the city.
He also spoke against granting Caroline £50,000 a year from the consolidated fund, suggesting it should be paid from crown revenue or admiralty droits, and argued her treatment showed the need for legal and parliamentary reform.
He supported George Canning's administration, but this left him at odds with his colleagues when he qualified his decision to vote against the repeal of the Six Acts by referring to his previous opposition when they were used to detain and prosecute Richard Carlile.
Other votes included favouring Catholic and Jewish emancipation, rejecting the Metropolitan Police Bill in 1829, and the Reform Act 1832, following which he was unsure whether to stand for Maldon or North Essex at the ensuing general election.