Ministerial office from the Lords
In almost her first debate she found herself on two sides of the same debate: discussing Clause no.23 amendment sponsored by Duke of Norfolk to Education Bill No. 2 1980. In June 1983, she joined the Lords Works of Art Committee. She became a resident expert on the committee, where she sat until 2010.
On leaving the UN post and entering the House of Lords, she was introduced as a Baroness-in-waiting from 1983 to 1985. Thatcher recognised her capabilities when she was given a ministerial post. Despite being in the Lords as Parliamentary Under-Secretary of State at the Department of Health and Social Security from 1985 to 1987, during which time she smoked cigarettes, and in spite of a deep concern for women's mental health with numerous charities, she would in perpetuum champion the cause of smokers to do what they wanted. A stubborn streak of wilful independence was typical of the Thatcherite in her, which she never lost up until her final verbatim speech in the Lords.
Moving to the Ministry of Agriculture, which suited her temperament better than social security, at the height of the Thatcher boom period from 1987 to 1989, she was made Parliamentary Under-Secretary. She was promoted to number two at the ministry in 1989 when the Prime Minister was still creating hereditary peerages; she valued her friendship and support. She continued until 1992, serving during John Major's administration as Minister of State at the Ministry of Agriculture, Fisheries and Food, when at age 69 she was the oldest female minister ever.
Her last role was once again as a Baroness-in-waiting to the Queen from 1992 to 1997, when a change of government ended her career. Acting in the capacity as a whip and a courtier, she felt compelled to attend at the bedside of Sally Mugabe in a London Hospital. She also received Mohammed Zahir Shah, the last king of Afghanistan, on behalf of the Queen. On several occasions during the 1990s, Lady Trumpington became acquainted with the New Labour opposition leaders, Tony Blair and Gordon Brown at Court. She was one of the few officials on hand in 1990 to recognise the new State of Mongolia, subsequently travelling to Ulan Bator to deal with KGB-backed Russian investors on a construction project.
Under the New Labour government Trumpington was created an Extraordinary Baroness-in-Waiting from 1998, given her many years of experience at Court. During the Labour government, she was consistently on the side of traditionalists. She voted often in favour of university tuition fees and raising the cap to £9,000; opposed constitutional reform, telling the Lords that she believed in the first-past-the-post system; and against the ban on fox hunting. Lady Trumpington was broadly in favour of Brexit, particularly the diminution of EU integration. She voted against the bill to make a referendum necessary to transfer powers back from EU to UK. Trumpington remained a principled opponent of soft measures on crime. Consistently supportive of tough measures, she was appointed Trustee of Crimestoppers in 2004. This tied in closely with ongoing visitations to Pentonville Prison, as Crimestoppers' emphasis was specific to schoolchildren and youth offending. She opposed "walking free" and community sentencing; her disarming charm when discoursing about conditions in Britain's jails alerted the Lords perspective on the significance of public participation in crime reduction initiatives.
Her sense of order, propriety and taste got her into trouble: mocked by a Labour peeress for commenting on the drab and dishevelled appearance of the House of Lords, she more than once appeared a martinet for insisting on "dress code". On another occasion she gave a V-sign to Lord King of Bridgwater in the House of Lords, 10 November 2011 when he referred to her advanced age during a Remembrance Day debate.
When she tried to debate the "plight of rural veterinary practices", the peeress pretended to be deaf: she had long railed against the most absurd forms of political correctness. In 2000–01, she was made President of the South of England Agricultural Show, taking the opportunity to promote animal health, a cause for which she had in 1995 been awarded an Honorary Membership of the British Veterinary Association, in response to her work in that ministry. Declining rural practices was caused directly by the government's farming policies; the yawning gap between policy and practice exposed weaknesses in the EFRACOM report. She had initiated the debate on 24 June 2004, and was widely praised for so doing. Throughout her career, she was notable for having raised uncomfortable truths about topics most politicians have avoided, such as the plight of women in prisons, or the fate of single mothers with mental health issues.
In December 2012, she acknowledged the campaign for the government to give official recognition to the work of Alan Turing. In a letter to the Daily Telegraph, the signatories, including Stephen Hawking, the physicist; Lord Rees, the astronomer royal; and Sir Paul Nurse, the president of the Royal Society, called on Prime Minister David Cameron to support a pardon for Turing's 1952 conviction for homosexuality. Trumpington worked at Bletchley Park during the war at the same time as Turing, commenting only on his presence.
She retired from the House of Lords on 24 October 2017, one day after her 95th birthday.