Lord Beaumont died in St Thomas's Hospital in London on Wednesday
The Green Party's only peer, Lord Beaumont of Whitley, has died aged 79.
He was chairman of the Liberal Party when he joined the Lords in 1967 but then defected to the Greens in 1999.
His opposition to the Liberal Democrats' support for an increase in free trade around the world was a factor in his decision to switch.
He was "a pioneering spirit in green politics" who would be "enormously missed", said Green principal speakers Caroline Lucas and Derek Wall.
They also praised him as "a passionate defender of human rights and civil liberties".
"He worked tirelessly to put sustainable development higher up the political agenda and, in particular, to promote sustainable energy," they said, in a joint statement.
Lord Beaumont died on Wednesday at St Thomas's Hospital in London, where he had been for several weeks, a party spokesman said.
Meanwhile the Green Party has launched its manifesto for the elections for London mayor and the London Assembly, both of which are staged on 1 May.
It has promised to cut 20p from all bus fares and the price of travelling on the tube off-peak.
It said it wanted 60% of all new homes to fall under the "affordable housing" banner and called for a minimum wage of £7.20 per hour.
Schemes to improve public transport and facilities for bicycles would be given greater priority than new roads under its plans, and there would also be a halt to the expansion of airports.
The party's mayoral candidate, Sian Berry, said she wanted to "transform London into a city that is both greener, more affordable, and a happier place to live in" with policies which she called "practical and effective".
The Greens are fielding candidates in each of the 14 assembly constituencies and have listed a further 11 people whom they hope will be elected via the second London-wide vote on the ballot paper.
At present there are two Green Party representatives on the 25-member assembly: Jenny Jones and Darren Johnson.
Mr Johnson said it was "widely acknowledged that the fact that more Londoners are cycling to work, insulating their homes and benefiting from the London living wage, has a lot to do with the work Greens have done on the London assembly".
"The more Greens Londoners elect to the assembly, the more we can get done," he added.