The stories are part of the Cthulhu Mythos and most had appeared in the magazine Weird Tales between 1939 and 1953.

Contents
The Mask of Cthulhu contains the following tales:

“Introduction”
“The Return of Hastur”
“The Whippoorwills in the Hills”
“Something in Wood”
“The Sandwin Compact”
“The House in the Valley”
“The Seal of R’lyeh”