The 25-year-old finished 50 points ahead of Austria's Ivona Dadic and Cuba's Yorgelis Rodriguez to take victory with 4750 points.
It ended a series of heptathlon disappointments after she finished fifth at last year's World Championships in London, sixth at the Rio Olympics and 28th at the World Championships in 2015.
Johnson-Thompson said trackside: "It's been up and down, a long old day, five events in one day is something I haven't done for a while.
"You have to believe it in order to do it, it's been a long time coming so it's so special to me."
She had not claimed a major medal since winning pentathlon gold at the European Indoor Championships in 2015.
Johnson-Thompson's triumph was also Great Britain's first World Indoor gold since 2014 when Richard Kilty took the 60m title.
Johnson-Thompson went in as favourite with the top three from last year's heptathlon at the World Championships - Nafissatou Thiam, Carolin Schafer and Anouk Vetter - absent.
She played down her chances before the Championships but finally made her global breakthrough in a weakened field.
A lifetime best of 12.68m in the shot put kept the Great Britain star in firm contention, despite failing to seriously impose herself in the morning session.
She only cleared 1.91m in the high jump, well below her personal best of 1.98m, and failed three times at 1.94m.
Johnson-Thompson also posted an unspectacular 7.36s in the 60m hurdles, even if it did equal a season's best as she came fifth in the heats.
She was 33 points ahead going into the final event, the 800m, but did finally dominate an event, winning in two minutes 16.63 seconds.
Earlier, Asha Philip missed out on the women's 60m final but Elliot Giles reached the 800m final and Laura Muir, fresh from a 3,000m bronze on Thursday, eased into Saturday's 1500m final.
Team-mate Eilish McColgan failed to join her after coming sixth in the second heat.
Muir said: "That was good, I obviously wanted to get that top two spot and that was a tough heat with Genzebe Dibaba in it but I got the job done so I'm really happy with it.
"Today I've been eating, seeing my physio and just relaxing as much as possible.
"I had the little mint chocolate from my hotel room last night and that was my 'little treat' but no I did the same as always."
