Paralympic hopeful Arnott competed in the T47 race at the Olympic Stadium.
The 19-year-old, who has limited movement in his left shoulder due to Erbs Palsy, clocked 11.32 seconds to finish fourth overall, first T46 competitor and first European. He was also the only under-20 athlete in the field.
Arnott’s previous best time had been 11.41 seconds, although he had run 11.29 wind-assisted at Lee Valley in April.
Poland’s Michal Derus won the race in 10.85 seconds, with Brazil’s Yohansson Nascimento second in 10.95 and USA’s Rodrick Townsend third in 11.26.
Meanwhile, Arnott’s City of Plymouth team-mate David King finished fifth out of eight in his 110m hurdles heat at the Anniversary Games.
King, though, was disappointment with his time of 13.77 seconds at the Olympic Stadium.
The 22-year-old, who represented Great Britain at the recent European Championships where he ran 13.54 seconds, said after the race: “Very disappointed with my run today. Just couldn’t get it right and hit hurdles. Now for a well-deserved break.”