Kate Middleton met Team Heads Together runners taking part in a final training session for the 2017 Virgin Money London Marathon.
The Dutchess of Cambridge joined them in placing a Heads Together headband on a Royal Mail post box on Wednesday.
She met a small group of Heads Together runners who came together for a final training session at Kensington Palace and thanked them for helping change the conversation on mental health.
Heads Together is the campaign she spearheads with The Duke of Cambridge and Prince Harry.
The campaign, in partnership with eight leading mental health charities, aims to end stigma, raise awareness and provide vital help for people with mental health problems.
Around 70 post boxes on the London Marathon route will wear Heads Together headbands, to match the headbands being provided to all 39,000 runners.
Charity of the Year for this year’s marathon wants to make it the ‘mental health marathon’ to get the country talking about mental health.
St Paul’s Choristers put headbands on to support the campaign, the former Olympic Stadium will illuminate a digital headband up to race day.
On Friday the South Bank will light up blue, with the National Theatre and the Coca-Cola London Eye matching the colour of the headbands.
Over the weekend, in support of the mental health marathon, the windows of the Virgin Money Haymarket Lounge and Planet Hollywood in Piccadilly will be dressed in a giant headband strip.
Later this week Heads Together will announce how everyone can join the campaign by wearing a headband online.
The Duchess of Cambridge dressed the Kensington Palace post box with a Heads Together headband alongside Royal Mail CEO Moya Greene.
Ms Greene said: “As a large employer, we recognise that we have a big role to play in tackling the stigma surrounding mental health.
“Although we have made some positive steps, there is still a long way to go. Like Sunday's race, this is a marathon.
"We need to build on the good work we have done so far, and that means helping to address this issue within the local communities that we serve every single day.”