Fellow Sherlock actor Martin Freeman, who plays Dr John Watson in the BBC One drama, had been vying with him for the prestigious honour.
Sherlock title actor Benedict Cumberbatch missed out on the Leading Actor gong, which went to Dominic West for his portrayal of Fred West in ITV1's Appropriate Adult. John Simm had also been nominated for the Leading Actor BAFTA, for the role of Tom Ronstadt in BBC One's Exile.
Sherlock lost out to Celebrity Juice in the YouTube Audience Award, but series co-creator Steven Moffat was given the 2012 BAFTA Television Special Award by Cumberbatch and Matt Smith in recognition of his outstanding creative writing contribution to television - see below (video via Radio Times):
Click here for the full list of winners at the ceremony, which was held at the Royal Festival Hall in London.