He was previously a partner in the Milwaukee law firm Gass Weber Mullins LLC, and served 8 years as a Wisconsin Circuit Court judge.

In 1999, Brennan was appointed by Wisconsin Governor Tommy Thompson to serve as a judge of the Milwaukee County Circuit Court.

Taking office in January 2000, he served in this position until November 30, 2008, presiding over almost 300 trials and thousands of motions in criminal and civil cases.

[2] Brennan returned to private practice in 2009 and became a partner at the law firm of Gass Weber Mullins, where he was a trial lawyer in the area of commercial litigation and handled appeals in federal and state courts.

[6] Upon the announcement of his nomination, Senator Tammy Baldwin criticized President Trump for bypassing the bipartisan commission composed to vet potential judicial candidates.

[2] He received a special commendation from the United States Department of Justice for his service as a member of the National Advisory Committee on Violence Against Women.