MacLachlan was born on March 3, 1938, in Cheyenne, Wyo. After receiving a bachelor's degree from the University of Connecticut in 1962, she worked as a junior high school English teacher. Her first published book, ‘The Sick Day', was published in 1979. ‘Through Grandpa's Eyes' and ‘Moon, Stars, Frogs, and Friends' followed in 1980.

MacLachlan's fiction tends to be realistic, with a notable exception being the fantasy book ‘Tomorrow's Wizard' (1982). Plots often revolve around children and their relationships with members of their immediate and extended families. ‘Arthur, for the Very First Time', which the Society of Children's Book Writers honored with a Golden Kite award in 1980, focuses on a boy spending the summer with his great-aunt and great-uncle while his mother is pregnant. In ‘Cassie Binegar' (1982), the main character laments her family's unconventional behavior, while the protagonist of ‘Unclaimed Treasures' (1984) struggles with the conventionality of her own family. ‘Mama One, Mama Two' (1982), the story of a young girl placed in foster care, was inspired by the author's extensive involvement with a family service agency.

‘Sarah, Plain and Tall' (1985) was recognized by The New York Times, School Library Journal, and others as one of the best children's books of the year. The novel tells the story of a pioneer man with two children who places an ad for a wife. A woman from Maine comes to their prairie home, but the children fear she may leave them to return to the East. MacLachlan later expanded the novel into a screenplay for a 1991 made-for television movie starring actress Glenn Close. A sequel, ‘Skylark', aired in 1993.

Among MacLachlan's other books are ‘Seven Kisses in a Row' (1983), ‘The Facts and Fictions of Minna Pratt' (1988), and ‘Journey' (1991). The author conducted creative writing workshops for both children and adults and served as a visiting lecturer at Smith College.