[2] She was born in Swansea, Wales and educated at Olchfa School, and graduated from Lady Margaret Hall, Oxford in 1984, followed by an LLM from the University of Virginia in 1985, where she was a Fulbright Scholar.

[1] Jefford was called to the Bar at Gray's Inn in 1986, became a QC in 2008, and was appointed as a Recorder in 2007.

[1] On 3 October 2016, she was appointed as a High Court judge, assigned to the Queen's Bench Division.

[2][3][4] She is the Presiding Judge for Wales and sits on the Technology and Construction Court.

[5] In 2016, Jefford was made a Dame Commander of the Order of the British Empire (DBE).