But the 64-year-old has now been told it is likely to be a precious 13th century weapon and was potentially carried by a knight.
The retired design and technology teacher, from Little Stretton, found the rusty blade in the man-made underground chambers near Beckbury in the late 1980s.
He only discovered its true origins when he decided to send the unusual object to Halls Fine Art Auctioneers in Shrewsbury to have it evaluated.
"I was living in Merridale in Wolverhampton at the time and was a keen walker," Mr Lawton said.
"They used to publish a walk in the newspaper every week and I gradually did the walks.
"There was one which directed you along this track, at the end of which were the caves, and it told you what to see.
"I saw this bit of the cross piece of the sword sticking out from the floor.
"It looked like a bit of rusty metal in lightly disturbed sand.
"I pulled it and out it came."
He said the sword had been in two parts and the tip was missing.
Mr Lawton said he took the sword home, adding: "I have never done anything with it. It has been sitting on my windowsill ever since."
It has since come to light the sword was found on private land, which Mr Lawton said he was unaware of at the time, so it is unknown who will have the legal claim over the object.
He has reported his find to the Portable Antiquities Team in Shropshire, who will do more research into how the sword ended up in the cave.
Militaria specialist Caroline Dennard, of Halls Fine Art, in Shrewsbury, said the sword was likely to be of 13th Century origin.
Ms Dennard said: "It's got all the hallmarks of a genuine sword from the 13th Century.
"It's got the right shape and it seems genuine.
"In populated areas knights were the only people authorised by the king to carry weapons so it is a probable assumption that this was owned and carried by a knight.
"Similar ones in excavated condition can make anywhere from £2,000 to £3,000. There's a condition issue with this.
"It's been broken at the tip and has probably lost eight inches."
Despite the broken tip, it is thought the sword could fetch between £1,000 and £1,500 at auction.
Peter Reavill, finds liaison officer for Shropshire, said: "Having just heard about this find it sounds like an amazing discovery and a story which needs some careful research.
"The role of the Portable Antiquities Scheme, a national British Museum based project, is to record archaeological artefacts discovered by members of the public.
"To do this properly takes time and we are just at the beginning of the research into this fascinating object.
"We hope that in the coming weeks we will find out more about the sword and how it made its way into the cave where Mark discovered it more than thirty years ago.
"Mark’s story of finding is brilliant and I can see why it is a treasured object for him and his family.
"It is the story of adventure and discovery especially given the caves interesting history.
"There are, however, some concerns as to how the find was made and its current legal status.
"All land, including caves, are owned by someone and it is possible that the landowner may have a claim on the find itself – unfortunately under the law there is no such thing as ‘finders keepers’."
The Caynton Caves are a series of man-made underground chambers in the grounds of Caynton Hall, near Beckbury.
Their original purpose and date of construction are disputed, though most authoritative sources date them to the 19th century.
The caverns comprise an irregular series of neo-Romanesque ambulatories and chambers hollowed out of sandstone, with carved archways, pillars, symbols and niches, apparently for candles.
They are located about 250 metres west of Caynton Hall, beneath privately-owned woodland, within a disused stone quarry.
One suggestion is that they were the result of quarrying during the mid-19th century and were then turned by the landowners, the Legge family, into a grotto or underground folly.
There have been speculative claims that the caverns are older, perhaps dating back at least to the 17th century, and some press articles have associated them with the Knights Templar.
However, historian and author Dan Jones considers that there is no evidence linking the caves to the Templars and Historic England dates the grotto as probably late 18th or early 19th century.
The caves are now closed due to safety concerns and vandalism.
Anyone caught on the land would be trespassing.