Herdman was born in Eastchester, NY, on February 11, 1948, the daughter of Raymond C. Herdman and Ellen (Saunders) Herdman. She recorded her debut album, The Water Lily, in 1976, and released it in 1977. (It has since been reissued on Rounder Records' Philo imprint.) The album had a distinctly Australian influence, containing musical settings of seven poems by Australian poet Henry Lawson (1867-1922) among its 11 selections, as well as a cover of Eric Bogle's "The Band Played 'Waltzing Matilda'," an antiwar song concerning the massacre of Australian soldiers at Gallipoli during World War I.

Herdman released her second album, Forgotten Dreams, on Flying Fish in 1980. In addition to familiar songs like the Depression-era standard "Brother, Can You Spare a Dime?" and covers of songs by well-known songwriters Randy Newman, Tom Waits, and James Taylor, Herdman also sang compositions by folksingers Bogle, Lui Collins, and Stan Rogers. In 1982, Herdman and her husband, Dick Hermans, an environmental activist, moved from Philadelphia to Pine Plains, NY. Her third album, 1983's Seasons of Change, had two more Rogers songs as well as selections by folksingers Si Kahn, David Mallett, Fred Small, and Judy Small.

In between its release and the appearance of her fourth album, Darkness into Light, in the summer of 1987, Herdman gave birth to a daughter, Suzanna, in 1985, and that influenced the recording of the LP, which Herdman acknowledged as "definitely my post-baby album," adding, "A lot of things which had been important became stronger feelings in me after giving birth." Selections included another Lawson poem, "When the Children Come Home," and a medley combining Malvina Reynolds' "Turn Around" with the traditional Irish tune "The Gartan Mother's Lullaby." Critics as usual were impressed, with Billboard's reviewer writing, "Folk singer Herdman's placidly beautiful voice demands attention...."

The influence of motherhood continued on Herdman's fifth album, an outright children's collection released by Alcazam! Records called Star Dreamer: Nightsongs & Lullabies that was released in 1988. In 1990, she teamed with fellow folksingers Anne Hills and Cindy Mangsen for the Flying Fish release Voices, putting on disc a trio that had been performing together for several years. A second Herdman children's album, Daydreamer, appeared on the Music for Little People label in 1993. In 1995, Herdman finally got around to releasing another solo album for adults, Forever & Always. In addition to featuring the work of her folksinging colleagues Hills, Mallett, Bill Staines, John McCutcheon, and Cathy Fink, it also included Billy Joel's "And So It Goes" and covers of "The First Time Ever I Saw Your Face" and "The Water Is Wide."

The trio of Herdman, Hills, and Mangsen reconvened for the holiday-themed live album Voices of Winter, released by Gadfly Records in October 1997, and Herdman's third children's collection, Moondreamer, appeared on Redwing in April 1998. The third Herdman/Hills/Mangsen trio album, At the Turning of the Year, was released by Hand & Hand in October 2000; it was another seasonal collection. In May 2003, Redwing released Herdman's sixth adult solo album and 12th album overall, The Road Home.