Co-written and first recorded by Leo Sayer (UK #2/IRE #3 1973).
Other hit version by Three Dog Night (US #4 1974).
From the wiki: “‘The Show Must Go On’ was written by Leo Sayer and David Courtney and first recorded by Sayer in 1973, becoming his first hit recording and was included on Sayer’s debut album Silverbird. The song uses a circus theme as a metaphor for dealing with the difficulties and wrong choices of life. Early in Sayer’s career, he performed it dressed and made up as a pierrot clown.
“The song was covered by Three Dog Night, whose version was released in 1974, and became the group’s final Top 10 US recording. In Sayer’s version, the last line of the chorus is ‘I won’t let the show go on’. Three Dog Night sang it as ‘I must let the show go on’, which Sayer was reportedly not happy about.”
Three Dog Night, “The Show Must Go On” (1974):