In 1951 he was appointed member of the city council of Randfontein and immediately elected deputy mayor and chairman of the finance committee of the city council. In 1953 he was elected mayor of Randfontein and two years later in 1955 was elected President of the TransvaalMunicipal Association. In 1957 he was elected mayor again and he was a member of the United Municipal Executive of South Africa for four years.
Parliament
Mulder was elected to the legislature in 1958 as a National Party member. John Vorster named him Information Minister in 1968. In 1977, he was implicated as namesake of the Muldergate Scandal, in which he was accused of having established a government slush fund for financing The Citizen, for the purpose of bolstering support for Vorster's regime among English-speaking South Africans. In January 1978, he was appointed Minister of Plural Relations and Development. Though the scandal eventually brought about Vorster's downfall and complete retirement from politics, Mulder lost the ballot to succeed him in September 1978. He barely lost in the first round to Pieter Willem Botha in a narrow 72-78 vote. In the second round, he lost with a comfortable 98 votes to 74, in Botha's favour. This was because 20 supporters of foreign minister Pik Botha threw their support to P.W. Botha, and only two to Mulder. Mulder was subsequently retained in Botha's reshuffling of the government. However, the Information Scandal stories started appearing in the media and this led to his resignation as Minister on 8 November 1978. Following his expulsion from the National Party in the wake of the Information Scandal, Mulder and his followers formed the National Conservative Party. However, all of the party's nine candidates were defeated at the 1981 general election. Mulder himself lost by 922 votes in the constituency of Randfontein, where he had won in the 1977 election with a majority of 7,763. After the founding of the Conservative Party, in which he participated, Mulder was elected to the House of Assembly at the 1987 general election, at the age of 62. By then suffering from terminal cancer, he died on 12 January 1988 before he was able to take his seat. His two sons, Pieter and Corné, became Conservative Party MPs in 1988 and currently serve in Parliament as members of Freedom Front Plus.