Mack McBride, played by John Reilly, was a character introduced in Melrose Place, the second series of the Beverly Hills, 90210 continuity, appearing in the season 7 episode The Younger Son Also Rises.
Mack was a great source of tension within the family. He had always favored Kyle and never considered Ryan very manly because he didn't serve in the Marines like him and Kyle, instead opting for college. At a dinner table with Kyle, Amanda Woodward, Ryan and Megan Lewis, he belittled Ryan and angered Megan to the point where they both walked out at dinner.
However, after Kyle told him that he was unable to have children, Mack turned on Kyle, calling him less than a man and saying that Ryan was his only hope to have grandchildren. Kyle and Mack came to blows during a poker game before Mack left to return home.