The double Olympic champion was always in control and when the pressure was on he applied his trademark acceleration to sprint away from Australia's Collis Birmingham, clocking 8:40.04.
But Farah admitted the race had been tougher than expected.
"My lungs were on fire and I was blowing for the first mile because it was pretty quick. It slowly eased off, but I still had to work for it and it wasn't as easy as I thought.
"There were thousands of people here wanting to support me and it was important that I kept going and motivated myself by thinking that I didn't want to disappoint anyone.
"As an athlete, the only real pressure is what you put on yourself. I want to win every race I go into and I wanted to keep that going throughout the whole year."
Chris Tomlinson eclipsed Olympic long jump champion Greg Rutherford with a best leap of 8.18 metres, while London 2012 finalist Lawrence Clarke finished third in a 110m hurdles race won by USA's Jason Richardson.
And Dwain Chambers continued his improved late season form winning the 100m by clocking 10.04 secs while USA's Wallace Spearmon won over the rarely run 150m distance.
Elsewhere, Holly Bleasdale finished second in the pole vault with a below-par best of 4.10m, the USA's Mary Saxer clearing 4.25m to win.
World 1500m medallist Hannah England, after her disappointing Olympics, settled for second over one mile, as she was just outsprinted by American Brenda Martinez.