Complex sentence: He remained Minister of Health in the government of Frost's successor, John Robarts until his resignation from cabinet in 1969 and oversaw the implementation of the Ontario Health Insurance Plan which is Ontario's version of Medicare.
Split into simpler sentences: He remained Minister of Health in the government of Frost's successor, John Robarts, until his resignation from cabinet in 1969. He oversaw the implementation of the Ontario Health Insurance Plan which is Ontario's version of Medicare.