Katherine Kiernan Mulgrew, or Kate Mulgrew, was born on April 29, 1955. She grew up in Dubuque, Iowa, the second oldest child (and oldest girl) in a large Irish Catholic family. When Kate expressed an interest in acting as a child, her mother, Joan, encouraged her to audition for local theater productions. Kate left Iowa for New York City at age 17 to pursue a career in acting. Kate was accepted into the Stella Adler Conservatory (part of New York University's acting program) and studied there for only a year, as she landed the lead role in the ABC soap opera Ryan's Hope in 1975, vaulting her to instant stardom.

At the same time she was filming Ryan's Hope, Kate played the role of Emily in the American Shakespeare Theatre's production of "Our Town" in Stratford, Connecticut. At age 23, following her success on Ryan's Hope, Kate was offered the lead role of Kate Columbo in "Mrs. Columbo," playing the wife of one of television's most beloved detectives, Lt. Columbo, as made famous by actor Peter Falk. While critically successful, the series was canceled after two seasons.

In 1981, Mulgrew co-starred with Richard Burton and Nicholas Clay in Lovespell, a film set in the era of Arthurian legend, as Irish princess Isolt, whose love story with Tristan is a classic tale of doomed love. That same year, Kate co-starred with Pierce Brosnan in the six-hour miniseries, The Manions of America, set in 19th century America just before the start of the Civil War. In 1985, she had a notable role in Remo Williams: The Adventure Begins as officer Major Fleming. In 1986, Kate appeared in a number of of Cheers episodes as congresswoman Janet Eldridge, a love interest for series lead Sam Malone (Ted Danson). In 1987, she appeared in Throw Momma from the Train as Margaret, Billy Crystal's ex-wife.