Why is St Martin "in-the-Fields"? I was reading about the Anglican church St Martin-in-the-Fields and was curious about the  in-the-Fields part of the name. It is named after Martin of Tours who was a saint in France during the 4th century.

Saints are sometimes referred to by a significant place (Kevin of Glendalough, Alipy of the Caves) or by a title (Anthony the Great, Pope Cyril I the Pillar of Faith). But here the saint is Martin-in-the-Fields, 
stylized as one word as if it is the name of the saint (think Kevin-of-Glendalough). Additionally, I've never seen an in description, going over a list of saints it's mostly ofs and thes. I would expect it to be St Martin of the Fields, if anything.

Why is Martin-in-the-Fields the chosen name what do the "Fields" refer to?

My speculation is that since he was a military saint, in-the-Fields means in the fields of battle, but I found no reference to that.
From the Wikipedia article you linked:


  St Martin-in-the-Fields is an English Anglican church at the
  north-east corner of Trafalgar Square in the City of Westminster,
  London. [...] Henry VIII rebuilt the church in 1542 to keep plague
  victims in the area from having to pass through his Palace of
  Whitehall. At this time, it was literally "in the fields", an isolated
  position between the cities of Westminster and London.


"In the fields" appears to be a reference to the location of the church, or the location of his burial site – not the name of the saint himself.