Danny Kruger, MP for Devizes, was photographed by a fellow passenger on the Hungerford to Paddington service, who then posted the image on Twitter.
They wrote: "Don’t blame it on the young people Boris when your own party aren’t even following your rules."
The MP was the prime minister’s former political secretary before his election as Devizes MP at the general election.
Face coverings have been compulsory on public transport in England since June 15 - a measure brought in by Boris Johnson's government.
Mr Kruger, David Cameron’s former speechwriter, has apologised and said he "simply forgot" to wear a face covering for the hour long journey.
In a statement, the MP said: “I boarded an almost empty carriage at Hungerford and quite simply forgot to put on my mask.
“When I got to Paddington I realised my mistake and covered up for the rest of my journey."
He continued: “If the person had reminded me rather than taking a photo and posting it on social media I would of course have put on my mask then and there.
“I do apologise for my mistake.”
On September 7, less than a fortnight before his own memory slip, Mr Kruger told his constituents in a Facebook message that they “MUST wear a face covering” unless exempt.
Great Western Railway, the train operator that runs the route, warns passengers on its website that they face a £100 fine for failing to wear a face covering on its services.
A number of rows have erupted during the pandemic with regards to MPs and ministers failing to follow government rules around the Covid-19 crisis.
Most notable was Dominic Cummings' 260-mile round trip to Durham during the strict lockdown, while the prime minister's key adviser had coronavirus symptoms.
Though Mr Cummings defended his actions, saying he had acted "reasonably", a report later found the furore had reduced people’s willingness to follow social distancing rules.