Kathleen was born in Marshalltown, Iowa.  She had two older brothers, Patrick and Michael, and a younger sister, Colleen.  She studied at Ottumwa Heights College and Norte Dame University and received her law degree from Catholic University in Washington, DC.  After working for a few years in Washington, Kathleen moved to Denver and began working for the Legal Aid Society of Denver. In 1985, she opened her own law firm continuing to represent low-income and marginalized persons with an emphasis on law reform cases.

Kathleen led the 25 year landmark Goebel case which led to court-supervised treatment programs that stopped Denver and the state of Colorado from ignoring mentally ill people. With a friend, Mary Ann Gleason, Kathleen founded the Colorado Coalition for the Homeless.

Ever the adventurist, Kathleen loved traveling the world with her travel buddy, JoAnn Slater.

A fierce and tireless advocate for the marginalized, Kathleen was respected by her peers. Her friend, John Holland, wrote the following tribute which captures Kathleen's spirit.

"The legendary Kathleen Mullen has passed. Well nicknamed the Queen by her colleagues, she ruled fiercely with Irish humor and the brave heart of a little sister of the poor.  Her causes were legendary and exceptional and she fought for them very hard, making a lot of bold differences. She was an exquisite race horse. A filly with unbelievable spirit. She ran her civil rights races for the people. Everyone she beat sang her praises however exasperated she left them as she flew by."

Kathleen died of Alzheimer's on June 27, 2025. She will be remembered as an amazing attorney and a faithful sister, aunt and friend.

Contributions in Kathleen's memory can be made to the Colorado Coalition for the Homeless , Rocky Mountain Immigrant Advocacy Network or Sisters of Humility , 820 West Central Park Avenue, Davenport, IA 52804.