The An Education actress exchanged vows with the Mumford & Sons star in a converted barn in Somerset, England and the groom's father John, who is a vicar, presided over the service, according to Britain's Mail on Sunday.
The venue was decorated with hay bales and Mulligan wore a full-length, backless ivory dress, while eight bridesmaids turned out in turquoise.
Guests included Hollywood stars such as Firth, Gyllenhaal and Miller, as well as Mumford's bandmates, who played at the reception in a marquee attached to a cowshed. Celebrity snapper Rankin was the official wedding photographer.
A source tells the Mail on Sunday, "Carey and Marcus wanted a low-key, rustic wedding. They thought their friend's farm would make a wonderful venue because it's so private."
The newly-weds began dating last year (11).