Race officials said Mackey and his 14-dog team were about three hours ahead of his nearest competitors, including Jeff King, Hans Gatt and Rick Swenson.
The Ruby checkpoint is more than halfway on the 1,150-mile race from Anchorage to Nome.
Mackey said in a report on the Iditarod Web site his run to the Yukon, which took about 14 hours, was "long but nice."
Despite his lead, Mackey said, he still did not think of himself as the front-runner in the grueling race.
Notable deaths of 2014 [PHOTOS]