Ambassadorship to Egypt and Religious minister in Sharif administrations
Since 1963, Zafar-ul-Haq had been participating in the national politics on a Pakistan Muslim League (PML) and is known for reflected views on religious conservatism while stressing to adopt humility to prevent violence and insanity. From 1980–81, Zafar-ul-Haq served as a member on the Panel of Experts for Pakistan Law Commission under the Ministry of Law and Justice. In 1981, he joined the Zia administration as the Information Minister and the Minister of Religious Affairs which served until 1985.
In 1985, President Zia-ul-Haq appointed him as the Pakistan Ambassador to Egypt which he tenured until 1986 when he was appointed as an adviser to Zia administration. He advised Prime Minister Mohammad Junejo on political and legal matters until 1987, and sided to join the Pakistan Muslim League (N) led by its President Fida Mohammad. In 1990–91, Zafar-ul-Haq presented Pakistan in the International Law Commission of the United Nations and was elected as Pakistan Senator for the first time in 1991 for a six-year term. In 1992, he was appointed in the Board of Trustees (BoT) of International Islamic University in Islamabad until 1997. During this time, he was appointed as the Secretary-General of the World Muslim Congress until 1997.
In 1997, he participated in the nationwide general elections on a Rawalpindi constituency which he successfully defended, and joined the second administration of Prime Minister Nawaz Sharif as the Minister of Religious Affairs. He played a crucial and vital role in drafting the Fifteenth Amendment to the Constitution of Pakistan to declare the "Islam as the Supreme Law"— the amendment was passed with the two-thirds majority in the National Assembly (lower house) but failed to be passed in the Senate (Upper house) in 1999.
Over the Kargil conflict in 1999, he reportedly warned the BJP ministry in India of "Pakistan resorting to the nuclear weapons issue if the country's territorial integrity, security and sovereignty is harmed. But his statement was not taken as serious as he was not involved in the national security matters at that time, and was seen as a political statement which he stated during the meeting with his constituents.
In 2000, Zafar-ul-Haq was elected as the Chairman of the Pakistan Muslim League (N), which he is currently serving, whilst Javed Hashmi became the President of Pakistan Muslim League (N).
On 4 December 2001, Zafar-ul-Haq appealed to the religious and conservative mass to call for the massive public demonstration against Pervez Musharraf, leading the pro-democracy movement with Amin Fahim to restore the civilian control. In 2009 he was elected to the Pakistani Senate for a six-year term. He was elected as the leader of the Independent Opposition Group (consisting of more than 6 parliamentary parties) in the Senate of Pakistan in 2009.
On 15 February 2018, PML(N) announced to appoint Zafar-ul-Haq for the Chairman Senate for the senate elections that were scheduled on 12 March 2018. Despite the PML(N) successfully retained the majority in the Senate, Zafar-ul-Haq's bid for the chairmanship was fell short through the number countings and, was defeated by independent Sadiq Sanjrani, who had support from the Asif Zardari and Imran Khan in spite of their disagreement.
Upon hearing the news of the number manipulation in the Senate elections, PML(N)'s leadership and the Prime Minister Shahid Abbasi reacted very negatively, and reportedly quoted in the news media that "Sanjrani holds no respect", and calls were made for the reelection of the Senate elections.
On 24 August 2018, he was appointed leader of the opposition in the Senate.