The snap showed the actress as her character Lady Sybil Crawley before her shocking death in series three. She was pictured inside a church when they filmed the episode her older sister Lady Edith being left at the altar on her wedding day by Sir Anthony Strallan.
WATCH: Jim Carter talks Downton Abbey film sequel
"Flowers being photobombed by adorable screen daughter @jessierbrownfindlay," he captioned the photo. "Lady Edith walked up the aisle that day… and back down again." Jessica was quick to respond, writing: "Oh Pa!!! Little Sibs. Always sporting a slight grin..." To which, Hugh remarked: "@jessierbrownfindlay coz you slight cheeky monkey [heart emoji]" [sic].
Fans also rushed to post comments, with one writing: "My favourite sister." Another said: "Sibs could brighten any day." A third fan commented: "Beautiful inside and out." Others were reminded of the sadness they felt when the character died of eclampsia.
"The sweetest soul! Broke my heart when her character died", wrote one fan, as another said: "I still can't watch lady sybil and Matthew's deaths, I always skip to the next two episodes."
Jessica got her breakout role in Downton back in 2010. She has since appeared in costume drama series Harlots, Jamaica Inn, Black Mirror and The Riot Club.
Last year, the 30-year-old opened up about the surprise success of the show and revealed why she departed after three series. Speaking on the Sunday Brunch show, she recalled: "It was the end of the first series and it was right as the last episode had aired, and I was in Soho.
The actress played Lady Sybil Crawley
"I was still at university, I was at art school, and I walked past a newsstand and there were the three [Crawley] sisters on the front of two or three different newspapers, saying that 14 million people had watched it. I had a big freak-out… I just thought it was this sort of quiet little period drama!"
"I was incredibly grateful for what Downton became and did, but it was my second job and I knew nothing about what I was doing," she later explained. "I really wanted to learn more. I had so much more I wanted to go and learn."
Like this story? Sign up to our newsletter to get other stories like this delivered straight to your inbox.