He first drew notice from theatergoers as a member of the trio that danced the Bob Fosse number "Steam Heat" in The Pajama Game (1954), and continued to hold their attention with the "Mu Cha Cha" number with Judy Holliday in Bells Are Ringing (1956).
A year later, he broke out of the chorus line and into choreography when he collaborated with Jerome Robbins on West Side Story, notably choreographing (without credit) a majority of the "America" and "Mambo" dance sequences.
[1] In addition to his theater chores, Gennaro worked steadily in television, appearing in and/or choreographing such shows as Your Hit Parade, The Andy Williams Show, Judy Garland's CBS variety program, and the Kraft Music Hall.
With his dance troupe, he was a guest on Ed Sullivan's CBS Sunday night variety show dozens of times, and was a member of the regular repertory company on the short-lived CBS variety show The Entertainers (1964–1965), which starred, among others, Ruth Buzzi, Carol Burnett, John Davidson and Bob Newhart.
[2] He also served for many years as choreographer for Radio City Music Hall, staging routines for The Rockettes.