Mark gives up his radical ideas and becomes the radio personality at Walden, going by the moniker "'Marvelous' Mark". His father, Phil Slackmeyer, goes to the college for a reunion, and is astounded that his son is taking fewer business classes so he can become a radio man. However, Mark has never really cared about his father's positions on things (having referred to his father as a "fascist" in early strips). They have always simply agreed to disagree. After graduating (in Doonesbury: A Musical Comedy, where his full name is revealed to be Mark Sheldon Slackmeyer), Mark goes to work for NPR, where he still works today, sometimes referred to as "'Microphone' Mark". His views remain liberal, but are less extreme than they were in early years (in which he even seemed to have communist leanings). Jewish by birth, Mark could now be best described as a secular humanist.
Andy Lippincott, who died of AIDS, came down from heaven as an angel in a "dream sequence" to tell Mark that he is gay. (Mark was portrayed as heterosexual in the '70s, though without much in the way of actual relationships. When he came out to an ex-girlfriend, she reveals that she was always suspicious. After this, his orientation ceases to be an issue for several years.) Mark takes it well, and forms a relationship with a gay political conservative named Chase. The two are united in a ceremony in Pago Pago, American Samoa by MacArthur, one-time assistant to then-Governor Duke. However, the two of them have more recently decided that they are not very happy with their relationship, and are thinking of getting a divorce, but are unable due to no documentation of the marriage taking place. In early 2007, Mark was forced out of the house by his partner Chase and began bitter rants of him on his radio show.