He was Singapore's Ambassador to Germany from 6 February 1998 to July 2003, and was concurrently accredited to Greece from March 2000 to July 2003. He then served as Ambassador to Belgium (from 22 August 2003) with concurrent accreditation to the European Union, the Netherlands (from 22 October 2003), Luxembourg and the Holy See. In this capacity, he represented Singapore together with Deputy Prime Minister and Minister for Law S. Jayakumar at the funeral mass of Pope John Paul II in the Vatican City on 8 April 2005, believed to be the largest gathering of heads of state in history. In 2006, the Vatican made Woon a Knight Grand Cross of the Order of St. Gregory the Great, which is conferred on Roman Catholic men and women in recognition of their service to the Church, unusual labours, support of the Holy See, and the good example set in their communities and country. Solicitor-General and Attorney-General Woon was appointed to the post of Second Solicitor-General on 3 October 2006, and Solicitor-General on 2 July 2007. In 2007, he was also made a Senior Counsel. Between January 2007 and 31 March 2010, he was a member of the Advisory Board of the School of Law of the Singapore Management University. In February 2008, Woon was appointed by the Ministry of Finance to chair a steering committee to review the Companies Act. Woon became Attorney-General on 11 April 2008. Three months into the job, he created some controversy when delivering an off-the-cuff speech at the launch of the Law Society of Singapore's Public and International Law Committee. He said: In mid-May, Woon commented that an acquitted person may not be guilty in law, but guilty in fact. Two months later, without referring directly to these remarks, Judge of Appeal V.K. Rajah wrote in a judgment that such comments could undermine confidence in the courts' verdicts and the criminal justice system, which was based on the doctrine of the presumption of innocence. The Minister for Law K. Shanmugam was asked in Parliament on 25 August 2008 to clarify the Attorney-General's comments. Shanmugam described the presumption of innocence as an "important and fundamental principle" which the Government was "absolutely committed to upholding". Nonetheless, it was "entirely possible for a person to have committed acts which amount to a crime and yet, there may be no conviction", as the trial process was designed to prove guilt