Bos was elected as a Member of the House of Representatives after the election of 1998, taking office on 19 May 1998 serving as a frontbencher and spokesperson for Finances. Bos was appointed as State Secretary for Finance in the Cabinet Kok II following the appointment of Willem Vermeend as Minister of Social Affairs and Employment, taking office on 24 March 2000. After the election of 2002 Bos returned as a Member of the House of Representatives, taking office on 23 May 2002 serving again as a frontbencher and spokesperson for Finances. The Cabinet Kok II was replaced by the Cabinet Balkenende I following the cabinet formation of 2002 on 22 July 2002. After the Leader of the Labour Party and Parliamentary leader of the Labour Party in the House of Representatives Ad Melkert announced he was stepping down as Leader and Parliamentary leader in the House of Representatives following the defeat in the election, Bos announced his candidacy to succeed him. Bos won the leadership election defeating former Speaker of the House of Representatives Jeltje van Nieuwenhoven and former Minister of the Interior and Kingdom Relations Klaas de Vries and was elected as Leader and Parliamentary leader and "Lijsttrekker" (top candidate) for the election of 2003, taking office on 19 November 2002. The Labour Party made a large win, gaining 19 seats and became the second largest party and now had 42 in the House of Representatives. For the Dutch general election of 2006 Bos again served as "Lijsttrekker". The Labour Party suffered a loss, losing 9 seats and now had 33 seats in the House of Representatives. The following cabinet formation of 2006 resulted in a coalition agreement between the Labour Party, the Christian Democratic Appeal (CDA) and the Christian Union (CU) which formed the Cabinet Balkenende IV with Bos appointed as Deputy Prime Minister and Minister of Finance, taking office on 22 February 2007. The Cabinet Balkenende IV fell on 20 February 2010 after tensions in the coalition over the extension of the Dutch involvement in the Task Force Urozgan of the International Security Assistance Force (ISAF) in Afghanistan and continued to serve in a demissionary capacity until the Labour Party cabinets members resigned on 23 February 2010. On 12 March 2010 Bos unexpectedly announced his retirement from national politics and that he wouldn't stand for the election of 2010 and approached Job Cohen as his successor.