She was elected as a councillor on Northampton Borough Council, serving from 1956–66, and became a whip. She unsuccessfully contested the parliamentary seat of Northampton in the 1959 and 1964 general elections for the Conservative Party. She was elected Member of Parliament for Birmingham Edgbaston in the 1966 general election, and held that seat in successive elections until she stood down at the 1997 election. The Conservative MP for Edgbaston, Dame Edith Pitt, had died on 27 January 1966 and it was the first time that a female Member of Parliament had been succeeded by another woman. Knight was a member of the Parliamentary Select Committee on Race Relations and Immigration, 1969–72. For more than two decades she was an active member of the Conservative Monday Club and was an outspoken opponent of the Irish Republican Army. Following the February 1972 Aldershot Bombing by the IRA she called for legislation to outlaw the IRA, and attacked supporters and sympathisers on the mainland. She was on the Select Committee for the Council of Europe from 1977, Home Affairs 1980–83, Lady Chairman of the Lords and Commons All-Party Child and Family Protection Group from 1978, on the Conservative Back-bench Health and Social Services Committee from 1982, Secretary to the 1922 Committee 1983–87. She was President of the West Midlands Conservative Political Centre 1980–83 and Lady Chairman of the Western European Union Relations with Parliaments Committee, 1984–1988. She served on the Council of Europe, and as Chairman, British Inter-Parliamentary Union. Knight was created a Life Peer as Baroness Knight of Collingtree, of Collingtree in the County of Northamptonshire in 1997 after standing down at that year's general election, and retired from the House of Lords on 24 March 2016. She was appointed MBE in 1964, and elevated to DBE in 1985. She was interviewed in 2012 as part of The History of Parliament's oral history project.
Knight, along with David Wilshire, introduced the Section 28 amendment to the Local Government Act 1988, which barred local authorities, including schools, from 'promoting' homosexuality. While promoting the new clause Knight claimed that children under two had access to gay and lesbian books in Lambeth, a claim which has never been substantiated. She linked discussion of homosexuality in schools to the spread of AIDS and described homosexuality as 'perverted' and 'desperately dangerous'. She has been described as a key force behind the legislation and a 'dedicated – not to say fanatical – anti-gay'. In June 2013, she opposed same-sex marriage, arguing that Parliament cannot change the fact that "marriage is not about just love. It is about a man and a woman, themselves created to produce children, producing children. A man can no more bear a child, than a woman can produce sperm, and no law on earth can change that. This is not a homophobic view. It may be sad, it may be unequal, but it's true." In the same year, she claimed it was wrong for David Cameron to apologise for the legacy of Section 28, while appearing to defend herself from accusations of homophobia by claiming that gay people are 'very good at antiques'. In 2018, when she was confronted about her role as an architect of and a main driving force behind section 28 she said she is sorry 'if' the law hurt anyone. Knight stated that her motivation had only been to maintain the welfare of children.
