[6] Gansler became involved with politics at 13, knocking on doors for Frank Mankiewicz, who was then running for the U.S. House of Representatives in Maryland's 8th congressional district.

[8] After working for a brief period as a real estate lawyer at a title company[6] and at the law firms of Coburn & Schertler and Howrey & Simon, Gansler was hired as an Assistant U.S. Attorney, serving from 1992 to 1998.

[10] Gansler also launched programs to fight gangs, punish criminals, and protect the public, including domestic violence dockets and drug courts, gang prosecution unit, Elder Abuse Task Force to target criminals preying on seniors, and an Internet crime unit.

[19] Gansler fought various forms of fraud that threatened home ownership, contributed to the cost of healthcare, and undermined citizens' financial security and well-being.

This work included the largest recovery in a healthcare fraud investigation in U.S. history against a major pharmaceutical company that illegally marketed drugs at the expense of consumer safety.

For example, he led a charge by 36 state attorneys general to demand accountability from the nation's largest provider of Internet services when it unilaterally changed its privacy policy.

[35] He also brought an action against a major retailer for an online breach that led to the theft of consumer credit card data from 100 million transactions.

[39][40] Gansler also established a statewide Internet safety initiative focused on school-aged children,[41] shut down an online anonymous gossip site being used to torment young students, helped eliminate the online illegal adult services section of Craigslist,[42] and worked on a nationally coordinated effort targeting sexual predators who use social networking websites to lure victims.

[45] He set up a Voting Irregularities Task Force to address voter suppression issues in Maryland,[46] and a Campaign Finance Advisory Committee to explore ways to ensure fair participation in the electoral process.

[52] In 2010, Gansler issued a legal opinion that clarified Maryland's ability to provide full faith and credit to valid out-of-state same-sex marriages.

[53] That opinion provided the underpinning for a subsequent Maryland Court of Appeals decision recognizing same-sex marriages from other states.

[63] Gansler formally announced his candidacy in the Maryland's 2014 Gubernatorial race to succeed Martin O'Malley, who was term limited, on September 24, 2013.

[67] Brown won the Democratic primary, but was defeated by Republican Party candidate Larry Hogan in an upset in the general election.

[70][71] On July 19, 2022, Gansler lost the primary, placing fourth behind Peter Franchot, Tom Perez, and Wes Moore.

[86] He mentored at-risk area youth, served on the Montgomery County Commission on Aging, and co-chaired the NAACP Criminal Justice Committee.

[87] For his work in establishing the Charm City Youth Lacrosse League, he received the “Innovator of the Year” award from Maryland's largest legal publication.