Born in Surrey, England, David Thornley took out Irish citizenship and was first elected to Dáil Éireann as a Labour Party Teachta Dála (TD) for the Dublin North-West constituency at the 1969 general election. He confronted the party leader Brendan Corish, who at the time of the Arms Crisis reportedly rejected out of hand any suggestion of military aid or use of force after the outbreak of violence in Northern Ireland.
In December 1972 he called for the immediate release of Seán Mac Stíofáin, then leader of the Provisional IRA. He was re-elected at the 1973 general election. In 1976 he voted for the Criminal Justice (Jurisdiction) Bill despite misgivings. He told The Irish Times: "When I get very depressed I drink too much. When I voted for the Criminal Justice (Jurisdiction Bill) I went on the batter for a forthnight [sic]." He lost his seat at the 1977 general election. In 1978 he joined the newly formed Socialist Labour Party stating that he had done so because: "There is no man in politics that I respect more than Noël Browne, despite our occasional differences. If the SLP is good for him, it's good enough for me".
Thornley died later that year (1978) in County Dublin, aged 42, from undisclosed causes.
The Trinity College Labour Branch is also known as the David Thornley Branch.