Same-sex marriage has been legally recognized in the District of Columbia since December 18, 2009, when mayor Adrian Fenty signed a bill passed by the Council of the District of Columbia on December 15, 2009. Following the signing the measure entered a mandatory Congressional review of 30 work days. Marriage licenses became available on March 3, 2010, and marriages began on March 9, 2010. The District became the first jurisdiction in the United States below the Mason–Dixon Line to allow same-sex couples to marry. 
In addition to recognizing same-sex marriages, since 1992 the District has also allowed residents to enter into registered domestic partnerships; since the passage of the Domestic Partnership Judicial Determination of Parentage Act of 2009, the District recognizes civil unions and domestic partnerships performed in other jurisdictions that have all the rights and responsibilities of marriage. The law gives the mayor discretion to recognize relationships from states with lesser benefits. 
Domestic partnership in the District used to be open to both same-sex and opposite-sex couples. One of the unusual features of the original bill establishing domestic partnerships was that it allowed partnerships to be created between people who were related by blood (e.g., siblings or a parent and adult child, provided both were single). All couples registered as domestic partners are entitled to the same rights as family members and spouses to visit their domestic partners in the hospital and jail and to make decisions concerning the treatment of a domestic partner's remains and estate after the partner's death. 
The measure also grants District of Columbia government employees rights to a number of benefits. Domestic partners are eligible for health care insurance coverage, can use annual leave or unpaid leave for the birth or adoption of a dependent child or to care for a domestic partner or a partner's dependents, and can make funeral arrangements for a deceased partner. 
The original bill establishing Domestic Partnerships in the District of Columbia was known as "The Health Benefits Expansion Act". It was passed by the City Council and signed into law by the Mayor of Washington, D.C. The bill became law on June 11, 1992. Every year between 1992 and 2002, the Republican leadership of the U.S. Congress added a rider to the District of Columbia appropriations bill that prohibited the use of federal or local funds to implement the Health Care Benefits Expansion Act.  The law was finally implemented in fiscal year 2002 after Congress failed to add the rider to the appropriations bill. 
Since the 2002 implementation of domestic partners, the benefits attached to domestic partnerships has been expanded many times. In the Health Care Decisions Act of 2003 domestic partners were given the right to make health care decisions for their partners.  The Deed Recordation Tax and Related Amendments Amendment Act of 2004 provided equal treatment, like spouses, of domestic partners for the purpose of paying the deed recordation tax.  Expanding benefits further, the Department Of Motor Vehicles Reform Amendment Act Of 2004 exempted domestic partners from the excise tax payable for transfer of title to your partner. And the Domestic Partnership Protection Amendment Act of 2004 amended the definition of the term "marital status" in the Human Rights Act of 1997 to include domestic partners. 
The Domestic Partnership Equality Amendment Act of 2006 was a major expansion of the benefits of domestic partners. The law came into effect on April 4, 2006. This act provides that in almost all cases a domestic partner will have the same rights as a spouse regarding inheritance, probate, guardianship, and certain other rights traditionally accorded to spouses. The act also gave the right to form premarital agreements for prospective partners, and for domestic partners to not testify against their partner in court. However, it does not extend most benefits of legal marriage to domestic partners, such as the marital estate tax deduction.  At the time of this latest expansion coming into effect in April 2006, there were 587 registered couples. 
The District of Columbia once again incrementally expanded the domestic partnership rights when in March 2007, the right to jointly file local taxes as domestic partners became law with the passage of the Domestic Partnerships Joint Filing Act of 2006. 
On May 6, 2008, the District of Columbia city council unanimously passed the Omnibus Domestic Partnership Equality Amendment Act of 2008. According to the Washington Blade, "the bill provides both rights and obligations for domestic partners in a total of 39 separate laws that touch on such areas as rental housing, condominiums, real estate transactions, nursing homes, life insurance, worker's compensation, investigations into child abuse and the police department's musical band, among other areas," thus "bringing the law to a point where same-sex couples who register as domestic partners will receive most, but not quite all, of the rights and benefits of marriage under District law." 
On May 20, 2009, the Domestic Partnership Judicial Determination of Parentage Act 2009 passed and signed into law allowing DC recognition of other states' domestic partnerships and amending DC laws on parentage entitlements and rights to children from adult domestic partnerships.  The proposed law became effective from July 20, 2009. 
Dean v. District of Columbia
On January 19, 1995, the District of Columbia Court of Appeals issued its ruling in Dean v. District of Columbia. In this case Craig Robert Dean and Patrick Gerard Gill, a couple who met all of the District's requirements for a marriage license except for being of the same sex, sought an order to compel the District to issue them a marriage license. The Court upheld a lower court decision denying them the license, finding that the District's marriage statute did not contemplate same-sex marriages despite being gender-neutral, that denying the license did not violate District law against discrimination based on sex or sexual orientation and that denying the license did not violate the Due Process Clause of the United States Constitution.
Recognition of out-of-state marriages
On April 7, 2009, the same day that Vermont legalized same-sex marriage, the council voted unanimously (12–0) to recognize same-sex marriages performed in other jurisdictions.  The move was hailed as a possible gateway to legislation of same-sex marriage in the near future. Under the District's procedures, the bill was voted on again on May 5, 2009, passing with a 12–1 vote. The act was signed by Mayor Adrian Fenty and was subject to a review period, which expired on July 7, 2009.
On June 13, the Board of Elections ruled that a petition seeking to repeal the law and delay its enactment until a vote was held in a referendum, would be invalid as it would violate provisions of the Human Rights Act which specifically disallow the public's voting against several protected classes—one being, sexual orientation.
On June 30, 2009, a D.C. Superior Court judge ruled against a group opposed to the new law, who wanted a referendum on the issue, and had also asked the Court to delay the enactment of the new law until the court decided the full case, and also allowed the voters to weigh in. The group had filed with the Court three weeks after the passage of the new law, Judge Judith E. Retchin ruled "there was no excuse" for them to file their lawsuit so late. She also agreed with the Board's decision that allowing a vote on the issue would violate the Human Rights Act.
Religious Freedom And Civil Marriage Equality Amendment Act 2009
On November 17, the District of Columbia Board of Ethics and Elections rejected a proposed ballot measure to ban same-sex marriage,  saying that the proposed ballot measure "authorizes discrimination prohibited under the District of Columbia Human Rights Act." 
On December 1, 2009, the act passed by a vote of 11–2 on its first reading. The second reading was voted on December 15, 2009 where the measure was again passed by a vote of 11–2. The bill received the mayor's signature on December 18 and had to survive a 30-day congressional review period before becoming law.  It was considered unlikely that the law would be overturned;  and the District government estimated that the law would take effect on March 3, 2010.  Marriage licenses became available on March 3, 2010. From that day onwards, the definition of marriage in the District of Columbia has been the following: 
Marriage is the legally recognized union of 2 persons. Any person may enter into a marriage in the District of Columbia with another person, regardless of gender, unless the marriage is expressly prohibited by § 46- 401.01 or § 46-403.
On March 25, 2010, during the Health Care and Education Reconciliation Act of 2010 debate, the U.S. Senate defeated an attempt by Utah Senator Bob Bennett  to "suspend the issuance of marriage licenses to any couple of the same sex until the people of the District of Columbia have the opportunity to hold a referendum or initiative on the question". 
Bishop Harry R. Jackson, Jr., the pastor of Hope Christian Church in Beltsville, Maryland, sued the District after the D.C. Board of Elections and Ethics refused to approve a ballot initiative on the issue of same-sex marriage. The Board stated that such an initiative would violate D.C.'s Human Rights Act. In January 2010, the D.C. Superior Court upheld the board's decision. 
On May 4, 2010, the District of Columbia Court of Appeals heard an appeal of the Superior Court decision. Attorneys for D.C. argued that D.C. Council acted within District laws in voting on and eventually passing the legislation. On July 15, 2010, the Court of Appeals upheld the Superior Court's decision in a 5–4 decision.  
The United States Supreme Court on January 18, 2011, rejected Jackson's appeal without comment.
A UCLA study concluded that extending marriage to same-sex couples would boost the District of Columbia's economy by over $52.2 million over three years, which would generate increases in local government tax and fee revenues by $5.4 million and create approximately 700 new jobs. Same-sex weddings are fast becoming a significant part of the wedding industry. Many wedding vendors are open to serving gay couples, but most do not advertise as such on their public websites. Select vendors in the DC area have come out enthusiastically in support of the gay marriage. 
A January 2010 Washington Post poll found that 56% of Washington, D.C., residents were in favor of same-sex marriage, while 35% were opposed.