Lord Dafydd Elis-Thomas, who represents Dwyfor Meirionnydd in the Welsh Assembly, has said he will not stand in the 2021 elections.
He has been a member of the assembly since its formation in 1999 and was its presiding officer from 1999 to 2011.
He was elected as an MP between 1974 and 1992, and was Plaid leader between 1984 and 1991.
After leaving Plaid Cymru in 2016, the independent AM was made deputy minister for tourism, sport and culture.
He has been a member of the House of Lords since 1992.
Lord Elis-Thomas told BBC Radio Cymru's Dewi Llwyd programme that he had had time to think and had realised during this period that there were other ways to serve society.
He said: "I have seen the value during this period of working in a different way. There's more to being a good citizen than being an elected politician for more than 40 years.
"And since I have reached that period of representing Meirionnydd at least for 40 years, it wouldn't make sense for me to stand in an election knowing that I would be 78 by the end of the next assembly term."
First Minister Mark Drakeford said it was an honour to have Lord Elis-Thomas in his cabinet.
I’d like to thank @ElisThomasD for the decades of enormous contribution he has made to Welsh life.— Mark Drakeford (@fmwales) April 12, 2020
It’s been an honour to have him spend his final assembly term as such a valued member of our government ministerial team. https://t.co/XIzXuzCQSI
Dwyfor Meirionnydd MP Liz Saville Roberts has tweeted her thanks to Lord Elis-Thomas for his service in representing the communities of Dwyfor, Meirionnydd and Nant Conwy as an MP and AM.
The Welsh Conservative leader Paul Davies said in a tweet that Lord Elis-Thomas had made a vast contribution to Welsh politics over the past decades.
He said: "His commitment and dedication has been second to none. His experience and expertise will be a loss to the National Assembly for Wales."