This church, designed by James Gibbs was completed in 1726.
Its patron is Saint Martin of Tours. Its design owes much to Sir Christopher Wren, with the walls and steeple being Wren-like, with the addition of a classical portico. A combination that influenced many later churches. (source: "London, and Architectural History" by Anthony Sutcliffe)
American CBS Journalist Ed Murrow famously broadcasted to America from the steps of St Martin-in-the-Fields during the Blitz. "U.S. listeners actually heard the people going by the church of St. Martin's-in-the-Fields on their way to shelters before a raid because Murrow laid his mike down on the sidewalk to pick up their unhurried footsteps" (Time Magazine Monday, Dec. 15, 1941.)
The steeple of the church features in William Hogarth's satirical engraving "Beer Alley" (1750). In Beer Alley all is prosperous and content, in contrast with its partner picture "Gin Lane", where everything goes to drunken wrack and ruin. Hogarth included the spire of St Martins as a symbol of good English taste. In contrast to this, "Gin Lane" shows the ornate Hawksmoor steeple of St George's Bloomsbury. See this link for a further discussion of these engravings.
Being close to London's Chinatown, the church has an active Chinese speaking congregation.
See also Wikipedia