He never really had the same interests as his brothers, and in the early seasons was often the victim (due to his naiveté) of some cruel joke that Brad and Randy had thought up on the spot for him. One of the pranks they would pull on him would be they told him he was adopted and told him they were aliens and so were his parents. In another prank, they made him eat a tadpole.

During adolescence, Mark began to adopt a more goth look and an anti-establishment kind of attitude. For a short time, he was learning to be a pilot and therefore he made fewer appearances in the episode. In some episodes, Mark was taking karate lessons also. While Mark's darkening demeanor worried Tim and Jill on several occasions, it never turned into anything extremely destructive--they even became more confident in themselves as parents, as they were never really shocked by anything he did. His gothic attitude was gone by the end of the series and he took back his regular attitude. Mark also grew a fondness in the areas of film production, music, and cooking (he was also into cooking in the earlier episodes). One 1993 episode shows Mark as a member of the Cub Scouts.