In 1955, Seeger continued her studies in the Netherlands, later traveling throughout much of Europe and even into Africa; that same year, she issued the Folkways 10" Folksongs of Courting and Complaint. In 1959 she settled in London, where she became involved with MacColl, the famed British musician and playwright. In the decades that followed prior to MacColl's 1989 death, the couple toured the world singing, lecturing, and preaching the importance of the British folk song tradition, typically emphasizing the connections between roots music and sociopolitical activism. Over time, Seeger's own original songs adopted an ardently feminist slant; she and MacColl also headed the controversial London Critics Group, producing an annual political theater production titled The Festival of Fools. They also operated and regularly performed at the folk venue the Singers Club and formed their own record label, Blackthorne; most important, however, was their work with BBC producer Charles Parker in developing the radio ballad, a groundbreaking musical documentary form combining field recordings of speech and sound effects with new songs in the folk idiom and complementary instrumental accompaniment.

From the mid-'50s onward, Seeger recorded regularly, cutting both original material and traditional compositions as a solo artist and in collaboration with MacColl as well as artists including Guy Carawan, Ralph Rinzler, and siblings Mike and Penny; among her key LPs are 1961's Two-Way Trip, 1973's At the Present Moment, 1977's Penelope Isn't Waiting Anymore, and the oft-released American Folk Songs for Children, an assembly of material originally collected by her mother. Seeger's best-known original compositions include "Gonna Be an Engineer," which emerged as an anthem of the women's movement, and "The Ballad of Springhill," penned about the Nova Scotia mining disaster. Seeger also wrote music for a number of films, television programs, and radio plays. After MacColl's death, she began working with the traditional Irish singer Irene Scott under the name No Spring Chickens, and together the duo formed a record label, Golden Egg. In late 1994, Seeger moved back to the United States, some four decades after first relocating to the U.K.; a year later, she completed work on the collections The Peggy Seeger Songbook, Warts and All and The Essential Ewan MacColl Songbook. Almost Commercially Viable followed in fall 2000. In 2003 Seeger released Heading for Home, the first of three volumes of recordings made with her two sons, Calum and Neill MacColl, in a cottage in rural England. The other two, Love Call Me Home and Bring Me Home, were released in 2005 and 2008, respectively. A live album in honor of Seeger's 70th birthday, Three Score and Ten, arrived in 2007.