Sgt Matt Harness, 36, said he had no choice but to deliver Spencer in the unlikely surroundings of the Waggon & Horses at Branston.
The drama began after he and his partner Kaylee had visited a labour ward when her contractions started.
The couple were told the birth was still some way off and returned home, only for Kaylee's waters to break.
Sgt Harness, who is a firefighter in Leicestershire and Royal Marines Reservist with the Nottingham detachment of RMR Merseyside, said they arranged to meet an ambulance at the pub after realising the baby was coming.
"She screamed its coming, its coming, - I had a look and there was a baby's head - so, I rolled my sleeves up, and before I knew it there was a baby in my arms," he said.
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Sgt Harness, who has completed tours of Iraq and Afghanistan, described delivering baby Spencer, who weighed 6lb 9oz, as "the most frightening experience of my life".
His partner Kaylee Harshaw, 28, said: "His birth certificate says Waggon and Horses car park, so when he is 18 he will quite like to say he was born in a pub."
She added: "He might not be quite so keen on his nickname - we called him Pickle because he was born in Branston."

The service at Holy Trinity Church was followed by a parade to Victoria Pier to cast wreaths into the Humber.
Keith Thompson, organising the parade, said during the conflict the Royal Navy requisitioned more vessels registered in Hull than from any other British port.
Trawlers, tugs and a North Sea Ferry, Norland, were manned by volunteers.
Argentina invaded the Falkland island in April 1982. About 650 of its troops, 255 British servicemen and three Falklands civilians died in two months of fighting.
The Norland Hull to Rotterdam passenger ferry landed Royal Marines and other forces in San Carlos Water and was often attacked by Argentine forces.
Mr Thompson, a Falklands veteran from the Norland, said in total nine Hull-registered vessels sailed - "the largest Falklands task force from one port".
Later in the conflict, the Norland transported prisoners of war back to Argentina and returned to Hull in 1983. She was scrapped in 2010.
During today's wreath-laying service on the pier, Hull firm SMS Towage was due to pull a tug down the Humber with jets firing as a mark of respect.
Mr Thompson said the public was invited to attend the service and the pier parade to lay flowers "for a loved one lost at sea in war or in peace time".
In the world wars hundreds of Hull trawlers and crews were also requisitioned by the Admiralty and used as minesweepers and patrol craft. Many merchant seamen were killed in action.
The memorial service took place at Holy Trinity, the nearest church to the port.

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30 January 2015 Last updated at 11:48 GMT
But visitors to Niagara Falls in North America got a shock when they saw adventurer Will Gadd climbing UP a frozen section.
Niagara Falls is actually a collection of three waterfalls - he made it up an icy part of Horseshoe Falls, the biggest one.
Will said: "I've been ice climbing for 30 years now and this is the coolest thing I've ever done."
Watch Hayley's clip to see Will in action.
A Royal Marine has told how he delivered his baby son in a Lincolnshire pub car park.
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A memorial has taken place to remember Hull's vessels used in the 1982 Falklands conflict.
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When you think of waterfalls, you tend to picture stuff going down...