Born in Cameron, Missouri, little is known about Allen's early years, education, and marriage to Sidney P. Allen. She attended the Lowthorpe School of Landscape Architecture between 1916 and 1919 in Groton, Massachusetts, under the name Beatrice Osborn Allen. In 1921 she visited gardens in England and Italy, including Gertrude Jekyll’s Munstead Wood. Between 1921 and the late 1940s Allen maintained a landscape architectural practice based in New York City, designing residential gardens primarily in the New York and New England area under the name Nellie B. Allen. Like many graduates of the Lowthorpe program, Allen excelled in the design of Anglo-American-style perennial borders, such as those at Dellwood, the John Henry Hammond estate in Mount Kisco. Other projects include the Isabel Dodge Sloane estate on Long Island, Three Waters estate in Gloucester, Massachusetts, Thornedale in Millbrook, New York, and the Anne Morgan estate in Mount Kisco. She was also a consultant for the Bishop’s Garden at the National Cathedral in Washington, and designed a parterre garden in collaboration with Constance Boardman for the 1939 New York World’s Fair. Her gardens were occasionally featured in House Beautiful, Landscape Architecture, and Country Life in America, but none of her gardens survive. A small collection of photographs, plans, and Lowthorpe School projects are held at the Carl A. Kroch Library, Cornell University.