A day earlier, the Guernseyman had led out Great Britain’s quartet as they qualified with a new national record of 3min. 11.95sec in Tokyo.
But here he was instead entrusted with finishing off GB’s campaign against other powerhouse nations, including the USA and the Dominican Republic, who were both disqualified earlier only to be reinstated via appeal.
The Sarnian hardly put a foot wrong individually.
He registered a 44.79 split – the fourth fastest of any competitor – as GB just missed the previous day’s record, clocking 3-12.07.
The top four teams were far out of reach when Chalmers took the baton in fifth.
In a thrilling dust-up, Poland sealed gold in 3-09.87 to beat the Dominican Republic, USA and the Netherlands.
Chalmers instead found himself in a skirmish with Belgium’s Kevin Borlee, one of the famed sprinting brothers, who eventually delivered a red-hot finish to record the day’s fastest leg and snatch fifth 0.56sec. ahead.
Despite being a previous World Championships finalist, Chalmers billed this ‘definitely the biggest race I’ve ever been in’.
‘To run on the last leg with these guys at the Olympics is just an amazing experience – something that I’ll definitely keep in my memory forever,’ he said.
GB’s medal contention had been hampered by a lagging start.
Chalmers’ surprise foil at the British Championships, Niclas Baker, could only muster 46.20 to finish at the tail of leg one.
The GB girls – with young Nicole Yeargin replacing Zoey Clark as Emily Diamond returned from the heat – had improved Chalmers’ position through solid mid-50sec. legs.
Chalmers’ sights are now on the men’s 4x400m relays, starting Friday.