an Irish-Mexican-American pianist and composer, best known as the accompanist to notably tone-deaf soprano Florence Foster Jenkins.

McMoon was born as Cosmé McMunn in 1901 in Mapimí, Mexico, the son of Maria (Valadez) and Cosme McMunn. His paternal grandparents were Irish and his mother was of Mexican descent. He moved with his family to San Antonio, Texas around 1911. He moved to New York City around 1920 to further his musical studies, and likely adopted the McMoon spelling around that time. Jenkins met McMoon sometime in the 1920s, and knowing McMoon was a concert pianist, eventually asked him to help her prepare for her performances and accompany her.

Apart from giving occasional piano lessons, McMoon never ended up making a career in music after Jenkins' death in 1944, and instead took an interest in bodybuilding and judging bodybuilding contests. He was a master chess player and was fascinated with mathematics. He resided in New York City until shortly before his death in August 1980. McMoon was diagnosed with pancreatic cancer and moved back to San Antonio, and died two days after arriving. His remains were cremated and his ashes rest at Sunset Memorial Park in San Antonio. McMoon never married or had any children.