Complex sentence: Before this he has worked with Carroll Winter & Co Chartered Accountants from 1959 to 1960 then with Peat Marwick Mitchell & Co in 1960 and became a partner with them from 1973 to 1979.
Split into simpler sentences: Before this he worked with Carroll Winter & Co Chartered Accountants from 1959 to 1960 and then with Peat Marwick Mitchell & Co in 1960. He was a partner with them from 1973 to 1979.