In ten years with the Environmental Defenders Office, Mark helped many hundreds of clients on a wide range of state and national planning, pollution, biodiversity or resource matters.

In 1999, he successfully represented the Conservation Council of South Australia in the State's longest ever environment trial, over tuna feedlots in Louth Bay.

His speech, and that of No Pokies MLC Ann Bressington which followed, were designed to highlight the lack of opposition by either major party to a bill changing the workers compensation scheme known as WorkCover.

Parnell denied his speech deserved the label of "filibuster" as he considered himself and Bressington responsible for presenting the entire opposing case.

See: 2008 Parnell–Bressington filibuster In February 2011 Parnell condemned policy decisions by the state Labor government such as approval of the Mount Barker redevelopment and the renewal of the uranium exploration company Marathon Resources' right to drill in Arkaroola sanctuary.