‘No-brainer’ is American in origin and was first used there in the 1950s. The earliest example that I’ve found of its use in the sense of ‘requiring little mental effort’ is in this ‘The Berrys’ cartoon, by Carl Grubert, which appeared in the Long Beach Independent, December 1959:
The first example of the term with the meaning of ‘easily made decision’ is from the Canadian newspaper The Lethbridge Herald, January 1968, in a report on an ice hockey game:
He’d break in on a goalie
and the netminder would make one of those saves that our
manager-coach, Sid Abel, calls “a no-brainer.”
The application of ‘no-brainer’ to individuals came later, after the term was already well established; for example, this from Tom Alibrandi’s 1979 novel Killshot:
“I’ll unanimously be voted the no-brainer-of-the-month award.”
See other phrases that were coined in the USA.