In the video, the man appeared to stumble a few times before collapsing off the stage, landing face first as several onlookers rushed to his aid.
As the guard was treated, the public walk-past was briefly halted.
As the shocking footage spread across social media, many expressed concern for the guard's wellbeing following the medical episode.
"I saw that happen live while screaming for the guard who was replacing him to grab the back of his jacket or something as he had become so unsteady just before the Changing of the Guards! I pray he is ok, because he collapsed on his face/head from the elevated step!♥️" one viewer wrote on Twitter.
Another added, "The poor soldier who just fainted while standing guard at the Queens lying in state was holding a rather large sword. It could’ve been very nasty!"
"I really hope that guard that either fell or fainted is ok?!" a third wrote.
It's not the only instance in which someone has collapsed amid the news of the Queen's death.
One royal mourner also fainted outside Westminster Hall as thousands gathered to pay their respects, with police and paramedics seen tending to the individual and carrying them out on a stretcher after witnesses saw them collapse ahead of the funeral procession's arrival.
Over the last week, hundreds of thousands of mourners have said their farewells to Her Majesty ahead of her funeral on Monday, September 19th.