Weems was born Katharine Ward Lane on February 22, 1899, in Boston, the only child of Gardiner Martin and Emma Louise (Gildersleeve) Lane, and received an elite education typical for wealthy women of her class. Her father was president of the Board of Trustees of the Museum of Fine Arts, Boston, and her grandfather was classicist Basil Lanneau Gildersleeve. She was named after her aunt, Katharine Ward Lane (d. 1893), who was a watercolorist. She studied art at the Boston Museum School under Charles Grafly and George Demetrios, and also at the summer studios of Anna Hyatt Huntington. Like many woman artists of the period, Weems often faced hostility because of her gender. However, she received support from two prominent female artists of the time: Huntington and Brenda Putnam, both of whom were working in New York.
