Middleton, who became a patron of the world-famous art gallery last month, received a private tour of the Lucien Freud exhibit before it opened to the public Thursday.
Middleton shined in gray tweed, A-line coat dress cinched at the waist, but her husband was notably absent from her side.
Prince William was deployed last week to the Falkland Islands to begin his six-week Royal Air Force search-and-rescue duties. Nevertheless, Middleton was poised and collected, according to the British papers.
In January, Middleton announced that she would be donating her time and support to an array of charitable organizations, including the world-famous art gallery, an art therapy and addiction charity, a group of children's hospices, and the Scout Association, the British equivalent of the Girl Scouts.
The Duchess of Cambridge is reported to have personally sifted through "a few hundred" potential charities before making her selection of which honorary positions to accept.
For Middleton, who was an art history major at the University of St. Andrews, where she met Prince William, the patronage at the National Gallery and a charity called The Art Room, which provides art therapy to children from five to 16, seemed like a natural choice.
She also threw her support behind the U.K.'s Patron of Action on Addiction, which aims to take a "dynamic and integrated approach to improving our understanding of addiction and our responses to it," and the East Anglia's Children's Hospices, which aid children with life-threatening diseases.
Middleton reportedly will visit an alcohol-free bar linked to Action on Addiction in the coming weeks, according to the Daily Mail.
Middleton's involvement and focus on giving back has prompted many comparisons to her mother-in-law, Diana, the Princess of Wales, who was dedicated to a host of philanthropic causes.
ABC News' Kevin Dolak contributed to this report.