Hopkin has expressed dissatisfaction with the material produced by Most, who had taken over as her producer with "Temma Harbour". Hopkin's final big hit was "Think About Your Children", released in October 1970, which reached number 19 in the UK. It was a worldwide hit, selling over a million copies. Produced by Mickie Most, "Knock, Knock Who's There?" was released as a single on 23 March 1970 and peaked at number 2 in the UK. In March 1970, Hopkin represented the United Kingdom in the 1970 Eurovision Song Contest, achieving second place with " Knock, Knock Who's There?" Although she gave a confident performance and sang in a crystal-clear voice, and despite being the pre-contest favourite, Hopkin lost to " All Kinds of Everything", performed by Irish singer Dana. Along with Donovan and Billy Preston, Hopkin was one of the chorus singers on the Radha Krishna Temple's 1970 hit single " Govinda", produced by George Harrison for Apple Records. In the US, "Temma Harbour" reached number 39 on the Billboard Hot 100 and number 4 on the magazine's Easy Listening chart. Her first single not to be produced by McCartney, it was released on 16 January 1970 and peaked at number 6 in the UK and number 42 in Canada. Hopkin's third single, " Temma Harbour", was a re-arrangement of a Philamore Lincoln song. She also expressed dissatisfaction with her manager at this time, Terry Doran. Hopkin said she interpreted "Goodbye" as McCartney pledging to stop " micromanaging" her career, since she was uncomfortable with his positioning of her as a pop chanteuse. It reached number 2 on the UK Singles Chart, number 13 on the Billboard Hot 100, and number 15 on the RPM chart in Canada. The next single was " Goodbye", written by McCartney (credited to Lennon–McCartney), and released on 26 March 1969. In the United States, Post Card reached number 28 on the Billboard albums chart.
It reached number 3 on the UK Albums Chart, although it proved to be her solitary success in that chart. It included covers of three songs from Donovan, who also played on the album, and one song each from George Martin and Harry Nilsson. On 21 February 1969, Hopkin's debut album, Post Card, again produced by McCartney, was released. That particular project did not materialise but Hopkin did sing the title songs to two of Baker's films, Where's Jack? and Kidnapped. In December that year, the NME music magazine reported that Hopkin was considering a lead acting role in Stanley Baker's planned film Rape of the Fair Country, which was to be based on Alexander Cordell's book of the same name. On 2 October 1968, Hopkin appeared at St Paul's Cathedral in London for the Pop Experience, where she sang " Morning of My Life", " Turn Turn Turn" and " Plaisir d'amour". It sold over 1,500,000 copies in the United States alone, and was awarded a gold disc by the RIAA. It reached number 2 on the US Billboard Hot 100, where for three weeks it was held out of the top spot by the Beatles' " Hey Jude", and spent two weeks at number 1 on Canada's RPM singles chart. Despite competition from well-established star Sandie Shaw, whose single was also released that year, Hopkin's version became a number 1 hit on the UK Singles Chart. Her debut single, " Those Were the Days", produced by McCartney, was released in the UK on 30 August 1968. The model Twiggy saw her winning the British ITV television talent show Opportunity Knocks and recommended her to Paul McCartney. She released an EP of Welsh-language songs for a local record label called Cambrian, based in her hometown, before signing to Apple Records, owned by the Beatles, one of the first artists to do so.
She took weekly singing lessons as a child and began her musical career as a folk singer with a local group called the Selby Set and Mary. Hopkin was born into a Welsh-speaking family in Pontardawe, Wales her father worked as a housing officer.