McCartney said the celebrity appeared "gobsmacked" more designers were not "joining forces" with her push for sustainability and ditching leather for eco-friendly materials.
DiCaprio chatted privately with Charles after they were separately given a guided tour by the designer of her innovative sustainable garments and accessories dubbed the "Future of Fashion".
The actor's appearance at Cop26 caused a buzz around the UN climate conference in Glasgow when rumours surfaced on Tuesday that he was attending, and the actor was finally spotted.
He is a climate change campaigner, working on a number of documentaries about animal poaching and the environment, and was appointed a United Nations representative on climate change in 2014.
In Glasgow's Kelvingrove Art Gallery a masked DiCaprio looked at McCartney's seemingly normal luxury handbags made from vegan, lab-grown mushroom leather and futuristic vegan football boots the designer created in collaboration with Manchester United star Paul Pogba and Adidas.
He left the main hall where the exhibition was staged before Charles arrived and after the prince viewed the items he was introduced to the film star in a quiet area of the gallery.
The celebrity quickly pulled off his face mask and shook hands with the royal who has spent the first few days of Cop26 in a whirlwind of speeches and meetings with everyone from Amazon founder Jeff Bezos to US President Joe Biden.
The actor was a guest of Ms McCartney's and the trio spoke together for around 10 minutes before Charles left and the designer joined a live discussion with industry experts about stopping the use of animal leather and fur.
Before taking her seat on stage, the designer said Charles found her exhibition "enlightening and inspiring".
She added: "I want to highlight my industry and call it out, we're one of the most harmful industries in the world to the environment and what I'm doing here is trying to provide sustainable solutions and technologies and a better way of doing things."
Commenting about DiCaprio's reaction she added: "He's just like 'I can't believe you're the only person in fashion doing this' and I think he's a bit gobsmacked I don't have more colleagues joining forces with me."
Earlier, Charles enthused about the joys of driving electric cars when he met McLaren's first woman racing driver.
The heir to the throne, who regularly uses electric vehicles to attend official events, met former rally driver Emma Gilmour, who will compete for McLaren in the 2022 Extreme E season, racing electric cars across some of the remotest spots on Earth.
Gilmour, 42, who was named as McLaren's first female driver in any type of motorsport yesterday, told the prince that she had only recently started driving electric cars. "They are amazing. The power is fantastic," she said.
Charles, 72, agreed and, laughing, said: "The acceleration is incredible. It runs away with you quite quickly."
The prince also staged a reception at Kelvingrove Art Gallery and Museum to mark his Sustainable Markets Initiative launching the Terra Carta Seal, with 45 companies receiving the recognition for making significant efforts to adopt sustainable markets.