Thomas Sangster was born in Southwark, a district in London on 16 May 1990 to Mark Sangster and Tasha Bertram. Sangster's father's family is from Banchory, Scotland. His great-grandfather,Anthony Bertram, was a novelist, and a maternal ancestor was politician and colonial administrator Sir Evan Nepean, through whom he has Cornish and Welsh ancestry. He has one sister, Ava Sangster. Sangster plays bass and guitar, and he learned to play left-handed guitar to portray the left-handed Paul McCartney in the feature film Nowhere Boy. He also learned how to play the drums in the feature film Love Actually (2003).