He was the eldest son and second of the six children of Eleonora and Reverend Thomas Watson.

[1] He was admitted to the Faculty of Advocates in 1851[1] and appeared for the defence of Dr Edward William Pritchard, the poisoner, in 1865.

Watson won the ensuing by-election[5] and was appointed Lord Advocate.

Watson married Margaret Bannatyne (1846-1898) in 1868, and the couple had five sons and a daughter.

Watson lived at 20 Queen's Gate in South Kensington, and was a member of the Athenæum and the Carlton Club.