The heartwarming clip was shared on social media after someone standing next to the 77-year-old filmed the procession.
Perreau, a star during the 1940s, caught Meghan’s eye as she went past, and caught her former student’s pleasant reaction.
In the clip, the fleeting sighting and accompanying squeak from Meghan causes Prince Harry to turn around for a moment to see what happened.
As the camera pans to Perreau, her delight can hardly be contained as she animatedly reacts to her special moment with the new Duchess.
The former actress trained Meghan in the craft long before the new Royal landed her coveted role in Suits.
American-born Perreau was a child actress who first appeared in Madam Curie aged just two.
She went on to find success in The Donna Reid Show and The Betty Hutton Show as an adult, with her work garnering her a star on the Hollywood Walk of Fame.
“Oh my God” screams Meghan as her old drama teacher Gigi Perreau manages to catch her eye in the crowd! Two seconds before I said do you think she’d recognise you! She was right! @firstname.lastname@example.org/6QMqLV1lp3
The actress-turned-coach was in London as a contributor for ITV’s wedding coverage.
Perreau was also interviewed about Meghan for the book: American Princess: The Love Story of Meghan Markle and Prince Harry by Leslie Carroll.
In the book, Perreau said she always knew Meghan would be “something special”, describing her as a “lovely girl and very hardworking.”
Meghan finally tied the knot with her Prince at St George’s Chapel yesterday.
The couple shared a touching kiss on the steps of the chapel before going on a carriage procession through the streets of Windsor.
Attended Eton College, followed by a gap year after graduating high school (including trips to Australia and South Africa), then studied at the Royal Military Academy Sandhurst.
Graduated from Immaculate Heart High School, a Catholic girls’ school in Los Angeles, studied theatre and international relations at Northwestern University School of Communication.
Spent 10 years in the armed forces, rising to the position of captain in the British military, served in Afghanistan twice, launched the Invictus Games - a Paralympic-style sporting event for injured servicemen and women.
Worked as a calligrapher while trying to make it in Hollywood, appeared as an extra on soaps including General Hospital and as a "suitcase model" on US game show, Deal or No Deal. Appeared in minor TV and movie roles before landing her career-making part as Rachel on legal drama, Suits.
Considered a serial playboy, dated a string of socialites including Chelsy Davy and Cressida Bonas, and was linked to British singer Ellie Goulding.
Married US film producer Trevor Engelson for two years from 2011-2013, dated Canadian celebrity chef Cory Vitiello from 2014 until 2016.
Trademark look is preppy and British, favours V-neck knits, checked shirts and freshly-ironed chinos, from brands including Gieves & Hawkes and Everlane. Has raised eyebrows for wearing stacked bracelets.
Signature style is sophisticated, classic and serious, favours luxury brands including Victoria Beckham, Jimmy Choo, Mulberry and Burberry.
Inspired by his humanitarian mother Princess Diana, Harry is a patron of The HALO Trust (against land mines), co-founded Sentebale (a charity to help orphans in Lesotho, southern Africa) and the Invictus Games, among others.
Global ambassador for World Vision, served as a counsellor for One Young World and is an advocate of the United Nations on gender equality and female empowerment.