Wall first became involved in the Green movement in 1979. He joined the Ecology Party (later the Green Party of England and Wales) in 1980. By 1987, Wall was standing for the Ecology Party against Chris Patten in Bath. At the time of the European Parliament election in 1989, Wall was one of three National Speakers in the Green Party. In the elections themselves, which saw the Green Party gain over 2 million votes (14.5% of the national poll), Wall received 15% of the vote in the Bristol constituency. During his time in the Green Party, Wall has been a Parish Councillor.
Wall rose to national prominence in the wake of the 1989 result, when he presented himself to the national press as a 'left wing' candidate for the ruling Green Party Council in opposition to the leadership. He styled himself a 'maverick' and a Green 'fundamentalist'. He was then in turn attacked as a 'parasite' by pragmatists such as Sara Parkin and Jonathon Porritt.. These divisions contributed to highly negative press coverage at the time.
Wall was also a prominent figure opposing the organisational changes by the group known as Green 2000 along with Penny Kemp and John Norris.. Organising a faction within the Green Party known as the Association of Socialist Greens, this left grouping was accused by pragmatists such as Mallen Baker as being responsible for ensuring the party was "going knowhere fast".
Derek's critique of Green Party politics is that ambition tends to dilute policy, reduce democracy and lead to failure. He says of the Green 2000 project to modernise the executive structures and reduce the number of Principal Speakers to two:
“The right around the Green 2000 faction wanted to make us into a mainstream party with mass appeal, ditch the radicalism, re-engineer the Party constitution and centralise power. We fought them. I remember Sara Parkin talking to the Independent about 'socialist parasites' i.e. myself and Penny Kemp who had been members nearly as long as her. They won and then imploded, when the Party received just a couple of percentage at the 1992 General Election. When the 'realists' believe in achieving a Westminster Parliamentary government by 2000 (thus Green 2000), give me fundamentalism.”
At the 2005 general election, Wall stood as a candidate for Windsor and received 2.5% of the votes. In November 2005, he was beaten by Keith Taylor in the election to be the Male Principal Speaker of the Green Party, by 851 votes to 803.
He is a founder member of the Green Left, an anti-capitalist and eco-socialist faction within the Green Party, which held its first meeting in June 2006.
Wall has continued to be an out-spoken member of the Green Party, particularly on the issue of entering into alliances with other parties. He was given a vote of censure by the Green Party Association of Green Councillors (AGC) when he made comments concerning the alliance between Green Party, Conservative and Liberal Democrat councillors on the Leeds City Council; he stated in
Red Pepper magazine that, "While I understand that repellent Labour councils may be the only substitute for alliances with Liberal Democrats and Conservatives, the fall in our Leeds West vote suggests that voters were displeased by a perceived shift to the right by Yorkshire Greens".
He was narrowly elected as one of two principal speakers of the Green Party of England and Wales in November 2006, alongside Sian Berry. The position of principal speaker was the closest role to that of leader within the Green Party until 2008.
In 2007 he was re-elected as Male Principal Speaker alongside Caroline Lucas, elected to the post of Female Principal Speaker.
He was prominent in the Green Empowerment group that campaigned against the creation of a single leader and deputy leader instead of the Principal Speaker system. He attacked the proposal for a referendum in Swansea:
I find the title "leader" embarrassing: it is so patronising, assuming a bunch of people have to be "led", the shepherd label that assumes the members are sheep.
Wall's critique centred on the link between the structure of leadership and ambition of the party; and the likelihood that a Leader coupled with ambition would be part of a project to move the party to the "right":
Political parties universally tend to attract those addicted to fame and careerism. ... Clearly, the biggest barrier to green electoral victory is the lack of proportional representation. Greens have been elected around the world in countries with PR. ... The fear is that a leader will shift the party to the right, cutting the Greens' radical edge. ...
The call for a single leader would compromise the party's commitment to radical democracy. It also suggests a failure to think about strategy in any real sense. The fear is that it could be part of a more extensive shift towards a new green politics of shallow environmentalism, rather than a thorough critique of an unequal, profit-motivated society.
He also linked the idea of a leadership structure directly to abuse of power. His September 2007 keynote speech took up this theme:
what is power? You can't touch it or taste it. Sometimes I think you can smell it, like wealth or sewage, and if it is piled up in one place it stinks like rotten cod. For it to smell sweet, we need to spread it around. For power to be creative, we need to spread it around.
Collecting power in one place is like collecting money in a mega billion bank account, the work of a miser, a misery, a self obsessed fanatic. Green politics is about giving everyone power.
At times, personal attacks surfaced in Wall's comments in the debate. Replying to a Guardian letter from Jonathon Porritt and others supporting leadership, Wall drew parallels with Porritt's own behaviour as a green 'leader':
Jonathon Porritt's call for the Green Party to select a single leader is flawed. Rather than suggesting the Green Party becomes a little more like the conventional top down political parties, Jonathon could show a little more personal leadership. Last year he took 42 flight clocking up air miles to go to meetings to tell people to cut CO2. I am calling on Jonathon to use video conferencing a little more and to fly a little less.
In the subsequent party-wide referendum, 73% of members polled voted to create a single leader. Wall said of the result, "The result of this referendum challenges the Party to create a leadership structure that is true to green ideals. It has put our future leaders on notice that the membership expects a more focused, more effective party, with a leadership team that is truly accountable to the membership in a real and effective manner".
Nevertheless in 2010 he declared himself a candidate for the post of deputy leader under the new system, losing to the incumbent Adrian Ramsay. During his Deputy Leader campaign, Wall continued to promote the view that a “right” wing of the Green party is attempting to subvert its democracy, agreeing that “talk of ‘modernising’ the Green Party [is] actually code for changing inner Party democracy and changing the politics of the Party”.. Nevertheless, Wall's opponents, the joint ticket of Adrian Ramsay and Caroline Lucas, continue to promote both reform of the party and a left-leaning politics. Derek's defeat was met with disappointment from Green Lib Dems, who cited his defeat as a loss of a “good source of ammunition against the [Green] party”.