The billboards rolled out this week across main London terminal train stations.
They have been created by production company Grand Visual and agency OgilvyOne UK, and will run on JCDecaux Transvision screens until mid-June.
Members of the public are invited to send an SMS containing their name to "London's Insider Tip Generator" to recieve a personalised message on a billboard.
The message will suggest ways they can make the most of their time in London. It will also be sent to the user in SMS form.
Melissa Weber, brand and communications director at American Express, said: "The outstanding service we offer, whether it is insider information on a destination, access to exclusive deals or comprehensive protection, enables our card members to get the most from their travels whether home or abroad.
"This campaign really brings to life our service in a creative, compelling and engaging way."
The activity forms part of American Express's long-running 'Realise the Potential' campaign.
The payment company also recently sponsored a six-part, branded-content series called 'Hidden Talent', which is currently screening on Channel 4.