He graduated from Nishna Valley High School before going on to attend the University of Northern Iowa, receiving a B.A.

Mark served as Vice Chair of the Iowa House Health & Human Resources Budget Subcommittee.

His other committee assignments of the 85th General Assembly include Human Resources, State Government, Veterans Affairs, and Labor.

Ernst's predecessor in the 12th Senate district, Kim Reynolds, also went on to be elected statewide as Governor.

[5] As of February 2020, Costello serves on the following committees: Ethics (Chair), Agriculture, Appropriations, Human Resources, and Veterans Affairs.