Met his then-wife Laura Haddock when both auditioned unsuccessfully for roles in My Week with Marilyn (2011). He was after the part of Colin Clarke that went to Eddie Redmayne and Laura had vied for the role of Marilyn that went to Michelle Williams.

Graduated from the 3 Year BA (Hons) Acting course at LAMDA. (July 2009)

While he is of mostly English descent, part of his father's family is American. His paternal grandfather was born in Connecticut, and his great-grandfather, Newton Cleveland Claflin, was from Revere, Massachusetts. Sam descends from Robert McClaughlin, a Scottish immigrant to Essex, MA in the 1600s (the surname was eventually shortened to simply "Claflin").

Good friends with actor and model Douglas Booth, with whom he co-starred in The Riot Club (2014).

Sam and his wife Laura Haddock welcomed their first child, a son called Pip Clafin on December 10, 2015. They got a second child, a daughter called Margot Claflin on January 6, 2018.

His father is an accountant/financial manager and his mother is a school pupil welfare adviser.

He is a ninth cousin, once removed, of American actor Ashton Holmes (through shared "Claflin" ancestry).