In May 2005 she was named vice president for the Northeast area, where she coordinated and integrated processing and distribution, transportation and delivery operations in that region.

In 2012, she began shutting down mail-handling facilities because of budget cuts brought on by less mail and congressionally-mandated pension-funding rules.

Postal Service's Board of Governors voted to appoint Brennan postmaster general to succeed Patrick R. Donahoe, who was set to retire in February 2015.

In May 2018, the Washington Post reported that President Trump personally pushed her to double the rates on Amazon and other firms.

[4] On April 27, 2024, Nativity BVM High School, from which Brennan graduated in 1980, inducted her into its Society of the Golden Cross.