Who is Mac from Cities of the Plain and what is their importance?

Mac McGovern runs the ranch where John Grady and Billy are employed. The government is set to commandeer his land, but Mac is determined to keep working until they do.

Mac has a deep appreciation for John Grady's talents with horses, and even though John is a very young man, Mac depends upon his advice when purchasing horses at auction. When John tells Mac he's planning to marry, Mac allows him him the use of a little house he owns in the hills. He also gives John his dead wife's wedding ring, since John can't afford to purchase a ring for Magdalena. In essence, Mac see John as a son.