Johnston's time of 4hr 33min 11 sec was close to a personal best, and she carved 14 minutes off her previous best combined bike-run time, her Wanaka coach, Val Burke, said.
Germany's Katja Schumacher was first in 4hr 22 min 46sec.
In her second European season as a professional athlete, Johnston was out of the water in third, sixth-fastest on the bike and fifth-fastest on the run.
Her result follows strong performances in New Zealand last summer, when she claimed the South Island Half Ironman title (4hr 32min 39sec), and finished second at both the Taupo Half Ironman in December (4hr 39min 49sec) and in the long-distance Challenge Wanaka in January (10hr 03min 56sec).
Her next half-distance race in Europe is in Kraichgau, Germany, on June 14 before she races in the long-distance triathlon at Challenge Roth on July 12.
Wanaka's adopted Canadian triathlete, Luke Dragstra, was the 12th man to finish in Barcelona, in 4hr 11min 06sec.
Gina Ferguson, of Christchurch, was seventh in Ironman Austria 70.3 (half-ironman distance), also on Sunday, in a time of 4hr 36min 02sec.
