The Letterkenny athlete was fifth in his heat in 1:46.69 but did enough to make Sunday's semi-finals in Beijing.
English, who needs a top-two finish to qualify automatically for the final, said he was "delighted" to make his first global semi-final.
A back injury severely inhibited English during the early stages of his summer campaign.
The 22-year-old UCD runner finished out of the medals at the European Under-23 Championships in the early part of July.
However, he showed a return to form as he mixed it with several of the world's best when he took fourth at the London Anniversary Games with his time of 1:45.49 also a qualifying mark for next year's Olympics.
That time was .65 outside his personal best set in 2013.
"I knew if I got boxed in and was sitting any further back than fourth I'd leave myself too much work to do so I wanted to have no regrets," English added after his Beijing heat.
Irish 400m hurdles star Thomas Barr missed out on a place in the final after finishing fourth in his semi-final.
Waterford man Barr, 23, ran 48.71 on Sunday, an improvement on his 49.20 in Saturday's heats.
Mark English has progressed to the 800m semi-finals at the World Championships as a fastest loser in the heats.