“I can’t believe it. I’ve dreamt of this for so long. To come here and do it at a home champs, it was something I dreamed of doing last summer, so I’m so happy I got to rectify that,” she told the BBC.
“It’s been a wobbly day but I’m happy I grasped the opportunity. All my family here and that’s something that never normally happens.
I went through a hard year last year, towards the end, and I just wanted my family to see me actually achieve something. They were here for every event – I still can’t believe it. Hopefully this can be the start of something.
Johnson-Thompson, who finished sixth in the heptathlon at Rio 2016 and fifth at last year’s World Championships in London, led the pentathlon going into the 800m – the event’s final discipline.
A time of 2:16.63 saw the 25-year-old finish the 800m in first, while nearest rival Dadic came third, 1.19 seconds behind Johnson-Thompson.
Earlier in the day, Johnson-Thompson was sixth quickest in the 60m hurdles, came out on top in the high-jump with 1.91m, and secured a personal best in the shot put, throwing 12.68m.
Johnson-Thompson recorded 6.50m in the long jump to ultimately place first going into the 800m, with a narrow 37-point advantage.
Gold in Birmingham will be a welcome relief for the Liverpudlian, having last taken to the podium in 2015 when winning the pentathlon at the European Indoor Championships.