Foss represented the Tukituki electorate for twelve years but was not successful in winning the electorate at his first attempt. In the 2002 election, he was beaten by Labour's incumbent, Rick Barker, by 6,410 votes.[4] He was ranked 47th on the National Party list, not high enough to enter Parliament as a list MP.[5]
Foss contested Tukituki again in 2005 and defeated Barker to enter Parliament for the first time.[6] He was returned for three further elections and his majority peaking at nearly 10,000 votes in 2011.[7][8][9] Foss voted in favour of the Marriage (Definition of Marriage) Amendment Act 2013.[10]
After the 2011 election, Foss was promoted to be a minister inside the Cabinet, as Minister of Broadcasting and Minister of Commerce.[14] He was also an associate minister in the education and ACC portfolios. As associate minister of education, Foss had responsibility for the ill-fated rollout of Novopay.[15] In January 2013 he was removed from the education portfolio and reassigned to be Minister of Consumer Affairs. In this term he also held brief acting appointments as Minister for Climate Change Issues, after the 2012 resignation of Nick Smith,[16] and as Minister for ACC, after the 2014 resignation of Judith Collins.[17]
National was elected for a third term in 2014, and Foss was appointed Minister for Small Business, Minister of Statistics, Minister of Veterans' Affairs, Associate Minister of Immigration, and Associate Minister of Transport, but no longer served in the Cabinet.[18] On 14 December 2016, following a change to the National Party leadership, Foss announced that he would retire from politics at the 2017 general election.[19] He relinquished his ministerial roles on 20 December 2016. The Tukituki electorate was won at the election by Lawrence Yule, who had succeeded Foss as the National Party's candidate.