King, at 51 the oldest female Great Britain Equestrian competitor, celebrated the 20th anniversary of her Games debut in Barcelona by thriving under pressure.
She was close to tears after being swept along on a tide of British support as 20,000 people packed Greenwich Park's main arena.
Imperial Cavalier's Dressage test earned a score of 40.90 penalties - his best score at four-star level - that left an elated King in third spot.
Had it not been for a poor final flying change, King could easily have taken the overnight lead from German Ingrid Klimke and Butts Abraxxas.
'The atmosphere was absolutely fantastic,' King said. 'I am not usually tearful, but coming into the arena with the crowd so behind me and being so good at keeping quiet when I came in was amazing.
'I knew if they all cheered and roared, Imperial Cavalier would have exploded and I would have found it hard to ride the test.
'Although they were really behind me, they were also ever so well behaved and kept nice and quiet until the end.
'We made a couple of errors, especially that last flying change when I lost him and he ran a bit to the right which made the change late, but most of his work was really pleasing.'
Day 1 of the Dressage ended with reigning European champions Germany underlining their gold medal credentials as 44-year-old Klimke led the field on 39.30, with team-mate Dirk Schrade second.
Current world and European champion Michael Jung rides tomorrow, and given his proven Dressage quality it is likely that Germany's three counting team scores after dressage will all be under 40.
And Australia also made their presence felt as Andrew Hoy, Sam Griffiths and Christopher Burton all finished in the top eight.