He joined Plaid Cymru in 1973, after being impressed by Emrys Roberts' campaign in the Merthyr Tydfil by-election. The following year in 1974, he began to work for Plaid Cymru as a parliamentary research officer. He stayed in the post until December 1979; it was a difficult year for the party, which had faced defeat in the Welsh devolution referendum and the loss of Gwynfor Evans' seat in the general election. With a reduced presence in Westminster the party had no need of a large parliamentary staff and Griffiths was made redundant.