I had a thought along those lines which most people probably wouldn't think about - Hendrik Verwoerd.
Assume he wins the race to replace Daniel Malan after he retires from South African politics in 1953 instead of succeeding J.G. Strijdom five years later, and assuming he is not assassinated, he would have to lead until 1975 to match the record. Considering South Africa's economic prosperity of the 1960s, that could be done. Verwoerd was much more willing to expand grand apartheid than John Vorster was, so the result could be more of the Bantustans gaining independence over the 1960s and 1970s. Vorster would probably be much more willing to back up Ian Smith, too, and also may also decide to formally annex South West Africa into South Africa.
Now, South Africa left the Commonwealth in 1961, but I think this would still count.