The double Olympic champion moved out of his comfort zone as he looked to hone his speed ahead of next month's World Championships in Moscow - and promptly became the sixth fastest man in history over the distance.
The 30-year-old, whose goal for the summer is winning the world titles over 5,000 and 10,000m, clocked three minutes 28.81 seconds at the Herculis Diamond League meeting in the principality, finishing second to Kenyan Asbel Kiprop.
It was some achievement, even by Farah's stratospheric standards.
His time eclipsed Spaniard Fermin Cacho's mark of 3mins 28.95secs, set back in 1997, and wiped out Steve Cram's 28-year-old British record of 3:29.67.
He is now the second fastest man in the world over 1500m - indeed he is ranked higher in this event than in either of the two in which he won Olympic gold last summer.
And all this came from a man who will next spring step up to the marathon, making his debut over the 26.2 mile distance in London.
Farah told BBC3: "I'm pleased, it was weird competing in a new event.
"Training has been going pretty well. The aim was to work on the speed. It would have been nice to get closer to Kiprop but he is a different class."
Cram, who commentated on the race, added: "To run like that is unbelievable. It puts him right up there in the top five or six of all time. I am shocked in a great way."
Farah would have been impressed by Edwin Soi's victory in the 5,000m as the Kenyan won in a world lead and meeting record 12:51.34.
Soi is the only man to beat Farah this year, defeating him in a 5,000m race in Eugene at the start of June, although on that occasion the Briton was hampered by illness.
Tonight's run, though, suggested the Kenyan will be Farah's main rival for gold in Moscow.
In a week which started with the sport engulfed in another huge drug scandal following the news of the positive tests from Tyson Gay and Asafa Powell, the second and fourth fastest men in history, it was perhaps inevitable that former drug cheat Justin Gatlin should win the 100m.
The American, who looks like being Usain Bolt's main rival for gold in Moscow, clocked 9.94 to finish ahead of compatriot Dentarius Locke (9.96) and Frenchman Jimmy Vicault (9.99).
Britain's 400m hurdles world champion Dai Greene gave his reaction to the race, writing on Twitter: "A bad end to a bad week for sprinting."
Greene himself had to withdraw from the 400m hurdles with a reported Achilles problem, worrying news with the World Championships less than a month away. The Welshman had looked to be getting back to form, running a season's best at the British Championships last weekend following a torrid start to the season.
In his absence Trinidad's Jehue Gordon won in 48.00.
Shara Proctor was third in the long jump with a best effort of 6.74m, while Tiffany Porter finished fourth in the 100m hurdles in 12.70, finishing ahead of Australian Olympic champion Sally Pearson.
Jessica Ennis-Hill had pulled out of the event as her ankle injury continues to delay her comeback.
Eilish McColgan was fifth in the 3,000m steeplechase, Steve Lewis sixth in the pole vault and Anyika Onuora sixth in the 400m.