A real estate agent, Mallett, 30, popped the question last Wednesday and shared the news on Instagram on Monday.
In the post, Spencer, 28, and her groom-to-be were seen beaming as they cuddled up and shared a smooch. Also on display was her glamorous ring -- a silver band complete with a healthy-sized glittering rock.
"So this was the best day of my life. 22nd of July 2020, I asked the love of my life to spend the rest of her life with me and she said YES," read Mallett's caption. "Couldn’t be happier and I love you with all my heart @ameliaspencer15."
Spencer left a sweet comment on the post as well.
"I couldn’t love you more," she wrote. "Happiest day of my life!"
Tatler reports that the two met during their time at the University of Cape Town.
Mallett, nephew of famed rugby coach Nick Mallett, proposed at the Clouds Wine and Guest Estate, which is known for its beautiful views of South Africa's mountains.
The Lady's Instagram account is private, but the outlet reports she also shared the news.
"I can’t wait to spend forever with you @gregmallett, I couldn’t love you more," she wrote, per Tatler.
"So happy for my daughter, Amelia, engaged to her boyfriend of 11 years, Greg - it’s wonderful to hear them both so excited about their future," wrote the Earl, 56. "Sending them both love, and every good wish for their life together. I love that Greg asked my blessing before proposing. Very sweet."
Spencer's sister Kitty also recently became engaged to her boyfriend Michael Lewis.