He remained senior partner with the firm of Monaghan, Marshall, Murphy and Watton until his election to the House of Assembly in 2003.
The PC Party won a majority government in the election, and Marshall easily defeated Mercer to become the MHA for Humber East.
Since his first election win Marshall has held a number of senior cabinet roles within the governments of Danny Williams and Kathy Dunderdale.
[10] In a 2009 cabinet shuffle Marshall was re-appointed Minister of Finance and President of the Treasury Board, and took the additional responsibility go Attorney General in 2012.
[14] After being sworn in as the 11th Premier of Newfoundland and Labrador, Marshall appointed Charlene Johnson as his successor at the Department of Finance and Felix Collins became the Attorney General.
[17] Marshall's 2010 budget projected a $194.3 million deficit for that coming year despite that he increased overall spending and cut personal and small business taxes.
[18] By the time Marshall delivered his 2011 budget the deficit he projected the previous year had turned into a $485-million surplus, due to higher than expected oil revenues.
[19] Marshall was sworn in as the 11th Premier of Newfoundland and Labrador on January 24, 2014, succeeding Dunderdale who resigned the post on the same day.
[22] When Williams announced his resignation as premier and Progressive Conservative leader on November 22, 2010, Marshall was considered a potential candidate in the race to succeed him.
In a Corporate Research Associates (CRA) poll conducted after Marshall became premier, support increased in these three areas.
[25] In the previous CRA poll conducted in November 2013, only 42 per cent of respondents were satisfied with the government's performance under Dunderdale.