Cobb was born in San Leon, Texas. After working as a crewman on a Gulf Coast shrimp boat, a construction worker and a waiter, Cobb graduated from the University of Houston Law School in 1993. After several years in private practice as a Houston, Texas attorney, he became engaged in politics. During the 1980s, he campaigned for the Democratic presidential candidacies of Jesse Jackson and Jerry Brown. Those experiences left him disenchanted with and disaffected from the Democratic Party. Consequently, he turned his activism to broad issues of democracy and corporations, joining with citizens' groups in lectures, seminars, and workshops throughout the U.S. He sought to promote his view that corporations became unelected governing institutions, which should be overthrown by means of a nonviolent democratic revolution.
In 2000, Cobb answered the call of Green presidential candidate Ralph Nader to organize Nader's Texas campaign. He coordinated a successful ballot access drive in the state. Concurrently, Cobb became Green Party of the United States General Counsel.
In 2002, Cobb ran for Texas Attorney General on the Green ticket and used his candidacy to "barnstorm" Texas localities with little Green representation. His election bid was unsuccessful, winning just 0.92% of the vote. The Green Party of Texas lost its ballot access, which remained out of reach until 2010. In 2003, a Green committee tagged him as a possible presidential candidate.
With the announcement in late December 2003 that Nader would not seek the Green Party nomination for president in 2004, Cobb became a front-runner for the nomination. On January 13, 2004, David Cobb won the first Green primary in the nation, that of the District of Columbia, beating local activist Sheila Bilyeu and several write-in candidates and gaining an early lead in the nomination scramble.
Nader eventually announced an independent campaign for president and sought the endorsement of the Green Party and other minor parties; his supporters continued to push for a Nader victory in the various Green Party primary elections in states across the country. Shortly before, the Green Party presidential nominating convention, held in Milwaukee, Wisconsin, in June 2004, Nader selected Green Party member Peter Camejo as his running mate. On June 26, on the second ballot, the convention selected Cobb as the Green presidential candidate – a process rocked by controversy as Nader had won the vast majority of actual Green Party votes in nearly all state primary elections (Cobb received only 12.2% support). The party also nominated Pat LaMarche as its candidate for vice-president.
Cobb stated his intention to run a campaign focused on building the Green Party and pursuing a "strategic states" or "smart states" strategy which would take into account the wishes of Greens in each state, and which otherwise would focus on states that traditionally are "safely" won by the Democratic candidate, or "safely" won by the Republican candidate, with a large margin of victory. Such so-called "safe states" are also referred to in campaign literature as "neglected states" because the Democratic and Republican candidates traditionally put most of their campaign energy into more competitive "swing states." Cobb's campaign said that each state's campaign would aim to follow the wishes expressed by local Greens. While some of Cobb's supporters urged swing state residents to vote for Democrat John Kerry in order to stop the re-election of President George W. Bush, other Cobb supporters encouraged votes for Cobb and LaMarche everywhere. The candidates themselves used the phrase "vote your conscience," campaigning both in swing states such as Wisconsin and safe states such as California.
On October 8, 2004, Cobb was arrested in an act of civil disobedience, breaking a police line while protesting the Commission on Presidential Debates for excluding third-party candidates from the nationally televised debates in St. Louis, Missouri. Also arrested was Libertarian candidate Michael Badnarik.
In the November 2004 presidential election, Cobb placed sixth in the popular vote total nationwide, earning over 119,859 votes (0.10 percent), but received no electoral votes. This represented a decline of over 90% support compared to the votes garnered by Nader.
After the 2004 election, Cobb and Libertarian nominee Michael Badnarik sought a recount of the Ohio vote and announced that they would challenge the 2004 presidential voting results in Ohio, even though neither challenger was claiming to have won the election, and even though Cobb had not even been on the ballot in Ohio. The challengers explained that it was an important matter of principle to make sure all the votes were counted accurately. They pointed to alleged voting irregularities.
Since running for president, Cobb's has become a member of the Board of Directors for the Green Institute, and of the Sierra Club's national Corporate Accountability Committee, a Fellow with the Liberty Tree Foundation for the Democratic Revolution, on the Steering Committee of Democracy Unlimited of Humboldt County, along with being the group's campaigns director, and is a Principal with Program on Corporations, Law, and Democracy (POCLAD).
Cobb facilitated the founding convention of the Green Party of Louisiana during a two-day convention which took place on August 31 and September 1, 2002, in New Orleans.
After moving to Eureka, California in 2003, Cobb also won a seat on the County Council of the Green Party of Humboldt County in a hotly contested election on June 6, 2006, a position he maintains.
David Cobb is currently a spokesperson for Move to Amend.
Cobb was the campaign manager for Green candidate Jill Stein in the 2016 U.S. presidential election.