Mary was born in Pontardawe, Wales to a Welsh-speaking family and began her musical career as a folk singer with a local group called the Selby Set and Mary. She released an EP of Welsh language songs for a local record label called Cambrian who were based in her home town, before signing to the Beatles' Apple Records. The model Twiggy saw her winning the British ITV television talent show Opportunity Knocks and recommended her to Paul McCartney. She became one of the first artists to record on the Beatles' Apple record label.

Her debut single, "Those Were the Days", produced by Paul McCartney, was released in the UK on August 30, 1968 (catalogue number APPLE 2). Despite competition from a well-established star, Sandie Shaw, who released her version of the same song as a single that same year, Hopkin's version became a number one hit in the UK singles chart, and reached #2 in the United States.

On 2 October 1968 Hopkin appeared at St Paul's Cathedral, London for the "Pop Experience" where she sang "Morning Of My Life," "Turn Turn Turn," and "Plaisir D'amour." On 21 February 1969 her debut album, Postcard, also produced by McCartney, was released.

It included covers of three songs from Donovan, who also played on the album, and one song each from George Martin and Harry Nilsson. It reached number three on the UK Albums Chart, although it proved to be her solitary success story in that particular chart.

The next single was "Goodbye," written by Paul McCartney (but credited to Lennon-McCartney), released on 28 March 1969 (APPLE 10); it reached #2 in the UK singles chart. It was kept off the top of the charts by the Beatles' single Get Back. "Goodbye" has never been officially released by the Beatles, although a demo version can be found on some of the Beatles' bootlegs.

She represented the United Kingdom in the 1970 Eurovision Song Contest singing "Knock Knock, Who's There?". Author & Historian John Kennedy O'Connor notes in The Eurovision Song Contest — The Official History, that she gave a very comfortable performance and sang in a crystal clear voice, but despite being the pre-contest favourite, Mary came second to "All Kinds of Everything", performed by Irish singer Dana.

"Knock Knock, Who's There?" was released as a single on 23 March 1970, reaching #2 again in the UK. Her second, folky album, Earth Song, Ocean Song, was released by Apple on October 1, 1971. The record was produced by her husband Tony Visconti and included cover versions of songs written by Cat Stevens, Gallagher and Lyle, and Ralph McTell.