During their time at the Inn, the duke and duchess met Dani Hope, one of the directors, as well as her children.
"What's remarkable about Action on Addiction is that it goes beyond helping those who are suffering on the courageous journey into recovery", the Duchess said.
Also at the event was Rolling Stones rocker Ronnie Wood, who has been open about his struggle with alcoholism, and his wife Sally, who were seated at top table with the duchess.
Bennett said the program, which taught her coping mechanisms and new behaviours, helped saved her life, following the death of her father. "And in some ways, it was the catalyst for my interest in early childhood development too", she continued.
Before the dinner - which was held at the stunning restaurant Spring, Somerset House - Kate met with Action on Addiction clients who are working as apprentices in the kitchen.
She added: "Sadly, for many who are suffering with addiction, they just don't receive the help they need early enough".
It was hard to distinguish between the looks of then vs now, if not for her change of accessories, choosing a pair of glittering Jimmy Choo heels and a Wilbur & Gussie foldover clutch last night, updating the look appropriately for a somewhat miserable summer's evening.
Kate spoke at the event, describing the charity as "very close" to her heart.
The $23,000-a-year establishment, which is about four miles south of the family's Kensington Palace home, is "a big, busy, slightly chaotic school for cosmopolitan parents who want their children to have the best English education money can buy", according to The Good Schools Guide. "They have already reached crisis point before they find the support they need", Catherine told the audience.
Action on Addiction is the only United Kingdom addictions charity that works across all the areas of research, treatment, family support and professional education.
It also helps others whose lives are blighted by addiction to gambling and other behaviours.
Has anyone in your family been impacted by addiction?
It also aims to lift the stigma attached to addiction.