The London carol service intended to "reflect upon the importance of love and empathy, and how much we need each other, especially in the most difficult times of our lives", according to Catherine's Kensington Palace office.
Each guest received an order of service illustrated by British artist Charlie Mackesy that asked, "how did I help?", with the reply, "you were by my side, which was everything".
Catherine attended alongside husband Prince William and their children Prince George, Princess Charlotte and Prince Louis.
The royal family inspected a "kindness tree" where people had hung messages on its branches in dedication to someone who has supported them.
The princess said in September that she had completed her chemotherapy treatment for an unspecified cancer and was looking forward to undertaking more engagements "when I can".
She attended Remembrance Day events last month and visited Southport in northwest England in October to meet people affected by a knife attack in July that killed three young girls.
Families affected by the attack were among the 1,600 people invited to the service, according to the BBC.
Catherine, wearing a festive red coat featuring a large black bow, mingled with some of the guests.
Among them were those "who have supported others in their communities, whether on a personal level with friends and family, through their work, or as they selflessly give up their time through volunteering," said Kensington Palace.
Six-time Olympic cycling champion Chris Hoy, who recently revealed he has terminal cancer, was due to light a candle at the event, which was to hear musical performances from UK singer Paloma Faith and US jazz performer Gregory Porter.
Each guest received a letter of thanks written by Catherine.
In it, she wrote that "this carol service is a heartfelt celebration of every one of you, and a reminder that at Christmas, and throughout the year, we must all shine for each other.
"Because in times of joy and sadness, we are all each other's light."
