The 26-year-old, also coached by Regan, produced an impressive negative split by going through the first 400m in 4:16 and coming home in the final 400m three seconds quicker.
Her winning time of 8:29.44 was a little more than two seconds outside the meet record and proved enough to comfortably hold off Venezuela's Pan American medallist Andreina Pinto.
"It was a strong meet for Lauren and I am pleased with her times today," Regan said. "In both swims - the 1500m and 800m - she finished strongly and comfortably. It is encouraging and the racing is a good litmus test of our progress.
"We don't have New Zealand swimmers winning many international swimming events over the years so that's great for them."
The pair will now join up with the rest of the Olympic squad in a training camp in Cairns this week.
Results from Santa Clara Invitational swimming meet in California today:
Men 100m breaststroke: Glenn Snyders (NZL) 1:01.44, 1; Kosuke Kitajima (JPN) 1:01.56, 2; Christopher Schurr Voight (MEX) 1:01.94, 3.
Women 800m freestyle: Lauren Boyle (NZL) 8:29.44, 1; Andreina Pinto (USA) 8:33.28, 2; Chelsea Chenault (USA) 8:37.28, 3.