The 59-year-old, who starred as Lord Grantham in the ITV period drama and the spin-off film, joined his co-stars to celebrate his on-screen daughter's big day in Chiswick.
Looking suave in an off-white suit and silk blue tie, Hugh appeared to be in great spirits as he mingled with the likes of Jim Carter - who starred as his trusty butler Carson in the beloved show.
It was undoubtedly a Downton Abbey family affair as most of the cast gathered to watch Michelle tie the knot with her beau, Jasper Waller-Bridge at St Nicholas' church in west London.
Laura Carmichael, who played Lady Edith Crawley on the BBC series, the sister of Michelle's character Lady Mary Crawley, was one of the bridesmaids at the ceremony. In photos released over the weekend, Laura could be seen wearing one of the sea-green gowns worn by members of the wedding party.
As well as Downton creator Julian Fellowes, stars Lily James, Joanne Froggatt and Elizabeth McGovern, a whole host of other celebrities were also in attendance including the groom's sister, Fleabag creator Phoebe Waller-Bridge and The Crown actress Lesley Manville.
Although Downton Abbey finished its sixth and final season in 2015, the cast reunited in 2019 and again two years later for the film spin-offs. During this whole time, the cast never lost touch.
"We never would've dreamed it would become the phenomenon the show has become and that 12 years later I'd be sitting here and we'd still be talking about it," Michelle said on The Reign With Josh Smith podcast last year.
The actress described returning to the franchise as like "going back home" to a family. "We are just delighted every time we get to get back together, put on those costumes and shoes again, and sort of slip into very familiar territory, which is quite rare for an actor to be able to go back (and) keep going back to something," she added.
"A lot of the time you are saying goodbye to something and then you are moving on to something quite quickly, a new group of people, a new territory, a different country. But whereas with this, it's like when Downton comes along, it's like, 'Oh, I know this, I know it so well and it makes it really easy actually just to step back into it.'"