Kaplan successfully argued before the Supreme Court of the United States on behalf of LGBT rights activist Edith Windsor, in United States v. Windsor, a landmark decision that invalidated a section of the 1996 Defense of Marriage Act and required the federal government to recognize same-sex marriages.

LGBT scholar and activist Aaron Belkin was Kaplan's high school friend and prom date.

[7] Kaplan joined the law firm Paul, Weiss, Rifkind, Wharton & Garrison LLP in 1992 and made partner in 1999.

[5] In 2017, Kaplan and co-counsel Karen Dunn filed a civil lawsuit on behalf of students, clergy members and local residents against 15 individual defendants and associated groups for damages following alleged injuries sustained at the 2017 Unite the Right rally in Charlottesville, Virginia.

[21][22] The lawsuit is based on the Ku Klux Klan Act[22] and according to The New York Times, the defendants are "an array of neo-Nazis, white identitarians and old-line pro-Confederates.

[12] The fund has raised more than $24 million to provide legal defense for sexual violence victims, especially those who experienced misconduct in the workplace and led 780 attorneys and 50 cases under way.

According to The Washington Post, Kaplan claimed "she intends to prove that Trump acted with 'malice,' meaning that he knew his statements were false or showed reckless disregard for the truth.

[31] Although the Department of Justice appealed that decision, Kaplan told reporters, "we are confident that the Second Circuit will affirm the District Court’s comprehensive and well-reasoned opinion.

The jury found in favor of Carroll on May 9, 2023, and awarded her damages of $5 million after finding Trump liable for sexual abuse and defamation.