Brennan's very early career was in the advertising business. Brennan moved to
Christie's in 2012. By 2020 she was chairman of business development of Christie's Americas; in this position she helped secure and manage major estate sales such as the 2018
Barney A. Ebsworth auction, as well as the collections of
Jayne Wrightsman and the non-profit
Cleveland Clinic. and during her tenure the region grew to account for 48 percent of Christie's sales. She played a key role in securing Microsoft co-founder
Paul Allen's $1.6 billion art collection for Christie's auction, She also championed increasing Christie's digital strategy and diversifying the auction house's offerings to engage younger and more diverse buyers. On February 1, 2025, Brennan was appointed chief executive of Christie's, succeeding
Guillaume Cerutti. ==Personal life==