Starting his career in Britain, he became drama critic for the *London Evening Standard*, and later, the *Sunday Times*, but after a number of notorious disagreements with the owners, he moved from the UK to the US. There, he was a columnist and theater critic for the short-lived *Hudson Review*, *The New Leader*, and *The New York Magazine*. Finally, he found a lasting home as drama critic for *New York*, a weekly magazine, where he published his last column on 13 December 1999.
During the 1960s and early 1970s, Simon became particularly famous for vitriolic criticisms of plays and films, earning the nickname "the hangman of Times Square." He was later labeled "the father of hate criticism." Later critics would attribute Simon's extreme statements as a sign of a critic in decline, or the desperate tactics of a man of lesser talent desperately trying to make a name for himself.
The publication of his 1967 volume of dramatic criticism entitled *Private Lives* further enhanced his reputation as an iconoclast. In this book, Simon coined the term "paratheatre" to describe many of the experimental works which were being produced at that time.
In the 1980s, he moved to Singapore, where he wrote an influential column in *The Straits Times* from 1982 to 1992, and then, starting in 1993, continued his career as a critic in Hong Kong, contributing a weekly column to the *South China Morning Post* that brought a western perspective on theatre to the region.