In 1968, Coetsee won the Bloemfontein West seat in the House of Assembly, vacated by J. J. Fouche, who became State President.
[2]: 61 He changed the legal system by introducing the small claims court and pioneering the Matrimonial Property Act in 1984.
This act had an important effect on the status of married women and introduced the accrual system of sharing property between spouses.
Coetsee also contributed to the process that made community service an alternative option to being jailed and did away with racially specific commissioners' courts.
He also walked the Indemnity Act through parliament, ensuring that those who took part in political negotiations after the unbanning of the African National Congress (ANC) would receive temporary immunity.