Upon graduating from law school, Green cofounded the law firm, Green, Wilson, Dewberry, and Fitch. In 1977, he was appointed justice of the peace in Harris County, Texas, and served until retiring in 2004. For nearly 10 years, Green served as president of the Houston branch of the NAACP. Under his leadership, the organization grew from 500 members to 3,500, expanded its budget, and acquired property to serve as its headquarters. He stepped down as president in 1995.

In 2004, Green, who considers himself a liberated democrat, "unbought, unbossed and unafraid" entered the congressional race for the newly configured Ninth District. Green ran on his long record of public service in Houston, emerging as the winner in the March Democratic primary with 66 percent of the vote. He prevailed in the general election with 72 percent of the vote.

In the current Congress, Green serves as the ranking member of the Oversight Subcommittee of the Financial Services Committee.

In his first term, Green introduced the Homes for Heroes Act. The bill created a veterans' affairs position in the U.S. Department of Housing and Urban Development to assist veterans experiencing homelessness to find affordable housing. "I am sorry to report, Mr. Speaker" Green said on the House Floor when the bill came up for debate, "that as many are sleeping in the suites of life, too many are sleeping on the streets of life." The measure was passed in 2016 when it was included in the Housing Opportunity Through Modernization Act and was signed into law.

Green became the first lawmaker to introduce and force a vote on three articles of impeachment against President Donald J. Trump in the 115th and 116th Congresses. When the House finally passed articles of impeachment in 2019 and 2021, Green voted for all of them.

Supported by his Texas Democratic Congressional Colleagues and both United States Senators from Texas, Green recommended Attorney Gregg Costa in 2011 as well as Federal Magistrate George Hanks and Harris County District Court Judge Al Bennett in 2014 for Federal District Court Judgeships. Additionally in 2014, Green recommended Judge Gregg Costa be elevated to the Fifth Circuit Court of Appeals. All three were nominated by President Obama and confirmed by the Senate to their respective benches.

In December 2020, Green's legislation establishing an Emergency Capital Investment Program became law as part of the 2021 Consolidated Appropriations Act. The legislation established a $9 billion Emergency Capital Investment Program (ECIP) for minority depository institutions and Community Development Financial Institutions serving low- and moderate-income communities.

In March 2021, Green's legislation reauthorizing and funding the State Small Business Credit Initiative was included in the American Rescue Plan Act of 2021. The legislation reauthorized and funded the State Small Business Credit Initiative at $10 billion. The funds expanded access to capital for small businesses still adapting to the pandemic and seeking to create high-quality jobs across the nation. The funding is expected to catalyze $100 billion in private investment for small businesses.

In November 2021, Green's legislation, H.R. 2689, the Minority Business Resiliency Act, was signed into law as part of the Infrastructure Investment and Jobs Act. The Minority Business Development Agency (MBDA) is the only federal agency dedicated solely to the growth of minority businesses. H.R. 2689 permanently codified the MBDA in federal law to promote and administer programs to assist the development and resiliency of minority business enterprises.

This Congress, Representative Green has introduced three resolutions designed to start healing the chasm of racial animus that exists within our nation today. The first of these is the Slavery Remembrance Day resolution, a resolution to create a commemorative day to remember the victims of slavery. The second is a resolution to remove the name of Richard B. Russell, a self-proclaimed white supremacist, from the Russell Senate Office Building. The third is a resolution to create a Cabinet-level Department of Reconciliation which would be tasked with eliminating racism and discrimination in the United States.

During his tenure as a U.S. Representative, Green has been successful in securing over $100 million worth of funding for community projects for the Texas Ninth Congressional District.