Millson married singer and Actress Caroline Fitzgerald in the summer of 1999. The two had two children, Jessica and Gabriel. In mid-2012, he dated Downton Abbey Actress Michelle Dockery for five months. In October 2012, it was reported in various media outlets that they had split. Millson met his current wife Sarah-Jane Potts in 2011 while they were co-workers on Holby City, the medical drama television series. The two wed on 31 December 2013. Millson has a stepson, Buster, from Potts' first's marriage to actor Tony Denman.