The rocker scaled the 3,560ft high mountain twice in one day to set his personal record and then played a gig in Warrington that night.
He said: "A few weeks into the Quid Pro Quo UK tour I grabbed the chance to make my 100th ascent of Snowdon.
"The only problem was my total climbs to date stood at only 98. The solution seemed obvious - climb it twice in one day.
"I have to admit my legs were a tad sore the next day but the show in Warrington that evening was most definitely rocking.
"Playing drums wasn't a problem but getting on and off the drum riser was."
Bandmate Rick Parfitt said Letley, who first climbed the Welsh peak when he was seven, was "mad of course".
He said: "It's an amazing achievement. I can't decide what is more crazy, climbing Snowdon 100 times, or doing it twice in one day."
The band, famed for their no-nonsense guitar style, celebrated more than 40 years in showbusiness at a Variety Club lunch at the Dorchester Hotel in London's Park Lane in October.