He was born in West Orange, New Jersey, the son of Willet Crosby Roper (1877–1966), an investment banker, and Florence Emily née Eveleth (1874–1961). His maternal grandfather William Hartley Eveleth (1840–1922) was the Superintendent of the college grounds for Harvard University and Radcliffe College. Roper received an honors degree in Fine Arts from Harvard University in 1933.
In 1952 he married Primrose Harley (1908–1978) an artist, daughter of Professor Edward Vaughan Berkeley Harley MD MRCP (1863–1923) and Mary Blagden (1869–?). Her paternal grandfather was George Harley and Ethel Brilliana Tweedie was an aunt. Primrose had previously married and divorced from Lt Col John Alfred Codrington (1898–1991), son of Lt-Gen Sir Alfred Edward Codrington
Roper had many garden commissions all over England, and some in Ireland (including Castlemartin), France, Italy, Switzerland and the United States. He was commissioned as Prince Charles' landscape gardener in 1981 to do the grounds at Highgrove House in the Cotswolds. From 1951 to 1957 was on the staff of the Royal Horticultural Society as Assistant to the Editor.
Roper wrote seven books, including Successful Town Gardening, Hardy Herbaceous Plants, The gardens in the Royal park at Windsor and On gardens and gardening.
The Lanning Roper Memorial Garden at the Trinity Hospice on Clapham Common, London, was originally designed by Lanning and constructed after his death. The woodland walk at the Farnsworth House in Plano, Illinois now bears Lanning's name, and at Scotney Castle in Lamberhurst, Kent in England, a streamside walk is dedicated to Lanning's memory.
Lanning Roper died in Paddington, London, and his ashes were scattered over the gardens at Scotney Castle.
A book about Lanning Roper titled Lanning Roper and his Gardens was written by Jane Brown in 1987.
- Roper, Willet Crosby (1931). The Problem of Pricing in a Socialist State. Cambridge, Massachusetts, USA: Harvard University.