This church's curious name has two origins: St. Martin was a Roman soldier who tore his robes in half to help dress a naked beggar on a cold winter's night. In his sleep, Martin saw a vision of Christ in the form of that beggar, and he promptly converted to Christianity. The "in-the-Fields" suffix was meant to distinguish the church from another named St. Martin located in the city at the time.
The church as it stands today was built in 1721 on the site of an older church built in 1542 by order of King Henry VIII, who wanted to redirect the funeral processions of plague victims away from his home at Whitehall. The modern church was designed by James Gibbs, one of the most influential architects in Britain. King George was reportedly so impressed by the church that he personally paid £100 to the men who built it. That's £100 total, not each. Gibbs liked the church, but not that much.
St. Martin-in-the-Fields's proximity to the Strand made it a popular church for the rich and upper class in the eighteenth century.