The ANC has always maintained an approval rating above 50% since the first democratic election in 1994, when Nelson Mandela became president.
But support for the party has fallen sharply amid anger over high levels of corruption, unemployment and crime.
The alliance between the centre-right DA and the ANC is unprecedented as the two parties have been competing for decades.
Under Nelson Mandela, the ANC led a campaign against the apartheid regime in 1994 and won the country's first democratic elections.
Critics of the DA have accused it of seeking to protect the economic privileges built up by the country's white minority during apartheid, a charge the party denies.
DA leader John Steenhuisen said this while addressing lawmakers in Cape Town late on Friday. “I think today is a historic day for our country and the beginning of a new chapter.”
The National Assembly was also sworn in as speaker by the ANC, while the vice-speaker position was transferred to the DA.
