Mercer directs the Mercer Family Foundation and served on the Executive Committee of the transition team of United States President-elect Donald Trump.  Mercer is the daughter of Diana Lynne (Dean) and billionaire hedge fund manager Robert Mercer.  During the 2016 U.S. presidential primaries, Mercer led Keep the Promise I, a Super PAC which was the largest outside benefactor of Ted Cruz.  Renamed Make America Number 1, it supported Donald Trump in the general election. She has consulted extensively with former Democratic strategist and pollster Patrick Caddell on campaigns.  She also worked with Stephen Bannon to create the film Clinton Cash .  In September 2016, Politico described her as "the most powerful woman in GOP politics." 
Under Mercer's leadership, the family foundation increased its donations from $1.7 million in 2009 to $18.3 million in 2014, with $35 million going to conservative think tanks and policy groups in that time period.
During the 2016 U.S. Presidential election, Mercer proposed creating a searchable database for Hillary Clinton's e-mails in the public domain and then forwarded this suggestion to several people, including Cambridge Analytica CEO Alexander Nix, who e-mailed a request to Julian Assange for Clinton's emails.   Assange responded to the report by saying he denied Nix's request. 
In the Trump transition, the Wall Street Journal reported that Mercer had helped in successfully lobbying against Mitt Romney as secretary of state and for Sen. Jeff Sessions as attorney general.  Politico reported that some said she had not favored Corey Lewandowski as possible Republican National Committee chair and that Lewandowski had reportedly resisted paying for services from data firm Cambridge Analytica –a company funded by the Mercers–early in the campaign, though a close associate of Mercer's denied the stories. Paul Manafort, Kellyanne Conway's predecessor as campaign director, was also said to be critical of Cambridge Analytica, which had worked for Ted Cruz and was financially backed by the Mercers.  Conway reportedly said that after the inauguration, the expectation was that Mercer was likely to lead an outside group, funded by her father, aimed at bolstering Mr. Trump’s agenda. It was assumed that Cambridge Analytica would also assist the group's efforts. 
In the 1990s, Mercer made three appearances on television, in the series Xena: Warrior Princess, Homeward Bound and Hercules: The Legendary Journeys, and work in the television film Jack Be Nimble.
Rebekah Mercer studied at Stanford University where she met Sylvain Mirochnikoff. The couple married in 2003.  In 2010, it was reported that they had bought six adjoining apartment units in Donald Trump's 41-story Heritage at Trump Place.  Mercer left her trading job to home-school the couple's children.  In 2016, Mirochnikoff, a native of France, was a managing director at Morgan Stanley.  In 2017, it was reported that the couple were divorcing.