Mr Mallett, 62, who found fame hosting children’s shows The Wide Awake Club and Wacaday in the 1980s, was drinking with friends in the garden of the Kings Arms in Cookham, Berkshire when the bike was taken from a locked-up area in the pub’s car park.
A keen cyclist, Mr Mallett had just returned from completing the Camino de Santiago in Spain riding the Giant E-bike he nicknamed Martin after his brother, who died last year.
He said the bike had been his “best pal” during the arduous trip, which took two months.
“Martin would have been 65 tomorrow and the loveliest thing would be to get the bike back tomorrow,” he said of his sibling, who had Down’s Syndrome and dementia.
Mr Mallett, who is offering what he described as a “substantial reward” for the bike’s return, said: “The irony is so awful. You can take it all over the world and yet at home it was not safe at the local village, in the lovely local pub.”
He said: “I’m heartbroken. It’s more than just a bike to me. This bike I’ve cycled with 2,500 miles, halfway across Europe.
“I left in the depths of winter and it took two months. I was on my own and my bike was my best pal along the way.
“I’m devastated to lose something so special.”
On Saturday he tweeted a picture of the bike with the message: “I’ve cycled half way across Europe on my Giant E bike carrying dear brother Martin’s memory. Securely locked in my local pub car park last night it was taken. CCTV shows them.
On Monday Thames Valley Police responded, also on Twitter, saying: “Hi Timmy. We are on the case. Please visit our website if you have any more information.”
“I’m not looking for retribution. I don’t know the circumstances of the people who took it. I’m appealing to somebody’s good nature to do the right thing,” the entertainer said.
“There’s a possibility that someone who knows where the bike is has enjoyed my TV shows over the years and would like to reunite me with it. Whoever took this had no idea how important it is and they could never have known,” he added.
Almost 100,000 bikes were reported stolen last year, according to Direct Line Insurance, amounting to 250,000 since 2015 and an increase of 17% on 2016.