During the 1980's Earnshaw taught European politics at Heriot-Watt University, and Hatfield Polytechnic in the UK. He worked in the Ministry of Defence in the period after the Falklands/Malvinas conflict in 1982.
After unsuccessfully standing as a Labour Party candidate in the 1989 European Elections he became a researcher for the Chairman of the European Parliament's Committee on Environment, Public Health and Consumer Protection, Sir Ken Collins. According to his biographical note 'he became recognised as an authority on the evolution and politics of European environment and health policy'.
He was subsequently Deputy Managing Director of the lobbying company GPC Market Access Europe before joining SmithKline Beecham (now known as GlaxoSmithKline) in 1995. Earnshaw was Director of European Government Affairs and Public Policy with SmithKline Beecham in Brussels. According to his biographical note he was 'instrumental in ensuring the adoption of the controversial biotechnology patenting directive in 1998; and he conceived the 2003 Regulation on tiered pricing/trade diversion.'
From drug industry lobbyist to Oxfam
Earnshaw left SmithKline Beecham in February 2001 after it merged with Glaxo Wellcome. In mid-April 2001 Earnshaw joined Oxfam to work on the campaign promoting developing countries gainer cheaper access to patented drugs. In his new job his pay packet would halve to £45,000 .
Earnshaw was critical of the industry for which he had worked for so long that embarked on a lawsuit against the South African governments use of generic versions of AIDS drugs."I find it quite frustrating that an industry which has so much to offer—like the pharmaceutical industry—is turning itself into an international pariah," he said.
"The pharmaceutical industry is a remarkably comfortable cocoon, and within that cocoon people find it terribly difficult to find new ways of doing things ... which is partly the reason, I suppose, that now they are being dragged kicking and screaming into change against their will," he said 
"It's basically the pharmaceutical industry against the world...Clearly the reputation of the industry is the lowest it could possibly be," he said.
In an interview with the UK newspaper the Observer Earnshaw said "Pharmaceuticals operate in a strange cocoon removed from people and the everyday marketplace. Change does not appear in its vocabulary. Executives have a very comfortable life but this can't continue any more.The reality is that this is the most unpopular industry in the world. Its reputation is at its lowest ebb and yet it has so much to offer to the world."
In an interview with Student BMJ, he described the drug industry strategy on the issue as "stupid". "My view on the access to medicines issue is that the industry has been very stupid. Crass stupidity is a good description. They have risked blowing everything they hold so dear - like their intellectual property protection - for what is a very tiny fraction of their revenue," he said.
"I have urged for a couple of years now for the need to think strategically about the issue and try and move to a high volume, low price paradigm, and I think the shock of this week's events in South Africa will now take them in that direction. Instead of going to court in South Africa, if they had invested a little bit in thinking about the problem, we would all have been a little better off," he said.
The strategy of providing high cost drugs for small numbers of people in the rich countries he described as both immoral and the wrong business strategy. "It is the wrong business strategy. It is out of place in the real world," he said.
While the Observer tagged Earnshaw as poacher turned gamekeeper, he extolled the virtues of compromise to the Student BMJ. "The only way the world is going to make any progress on this is through working together. If I can bring some expertise and ability to Oxfam and NGOs [non-governmental organisations] based on what I have done in the past, that's good, and I believe that more people should move from the corporate sector to NGOs and the other way as well."
... and back to the lobbying industry
In June 2002 Earnshaw was appointed as Managing Director of the Brussels office of Burson-Marsteller (BM). His biographical note describes him as Managing Director of the Brussels office of B-M's subsidiary lobbying company, BKSH.
Following calls by European Commission Vice-President Siim Kallas for greater regulation of the lobbying industry Earnshaw argued self-regulation was sufficient. "If you regulate strictly, the people who get hurt are the little people -- the people who do not have a voice -- not the people who can bend the rules," he told the Washington Times 
He also argued that the commission would have difficulty gaining agreement on defining who a lobbyist is: "If you are a Socialist, business is the lobbyist; but if you are a Christian Democrat, NGOs like Greenpeace are the lobbyists." 
- David Earnshaw and David Judge, The European Parliament, Palgrave Macmillan, May 2003. ISBN 0333598741
- Burson Marstellar David Earnshaw. Accessed 8 October 2014.
- Corporate Europe Observatory, "Industry and the EU Life Patent Directive", Corporate Europe Observer, Issue 1, May 1998.
- Stephen Beard, "Marketplace Archives, Marketplace, American Public Media, April 16, 2001.
- Helen Palmer, "Marketplace: News Archives", Marketplace, American Public Media, April 18, 2001.
- Nick Mathiason, "Gamekeeper turned poacher gets to work", Observer, April 22, 2001.
- Roger Dobson, "Drug company lobbyist joins Oxfam's cheap drugs campaign", Student BMJ, (undated) 2001. (BMJ is the acronym for the British Medical Journal).
- Gareth Harding, "Analysis: Reining in EU lobbyists", Washington Times, March 8, 2005.