Leonard Constant Lambert (23 August 1905 – 21 August 1951) was a British composer and conductor.
Lambert, the son of Russian-born Australian painter George Lambert, was educated at Christ's Hospital and the Royal College of Music. Lambert was a prodigy, writing orchestral works from the age of 13, and at 20 received a commission to write a ballet for Sergei Diaghilev's Ballets Russes (Romeo and Juliet).
For a few years he enjoyed a meteoric celebrity, including participating in a recording of William Walton's Façade with Edith Sitwell. Lambert's best known composition is The Rio Grande (1927) for piano and alto soloists, chorus, and orchestra of brass, strings and percussion. It achieved instant success, and Lambert made two recordings of the piece as conductor (1930 and 1949). He had a great interest in American Negro music, and once said that he would have ideally liked The Rio Grande to feature a black choir.
During the 1930s, his career as a conductor took off with his appointment with the Vic-Wells ballet (later The Royal Ballet), but his career as a composer stagnated. His major choral work Summer's Last Will and Testament (after the play of the same name by Thomas Nashe), one of his most emotionally dark works, proved unfashionable in the mood following the death of George V, but Alan Frank hailed it at the time as Lambert's "finest work". Lambert himself considered he had failed as a composer, and completed only two major works in the remaining sixteen years of his life. Instead he concentrated on conducting, and appeared at Covent Garden and in BBC radio broadcasts, and accompanied the ballet in European and American tours.