Garner came home as part of the second group of riders having helped Armitstead in the first half of the race before a breakaway group of seven, including the English duo of Armitstead and Emma Pooley, escaped.
With the big hitters from Australia and New Zealand also in the lead group, there was little desire from the following pack to chase it down as the three nations with the highest hopes of gold happy that the breakaway gave them a good shot of attaining that goal.
That left it to Pooley to assist gold medal favourite Armitstead for the final three laps of the Glasgow course. The time trial silver medalist performed admirably, leading them for long spells and quickly responding to any attempted attacks from the Australians.
In her last race before retirement, Pooley made a break on the final lap, forcing the Australians and Linda Villumsen of New Zealand to chase, setting it up perfectly for Armitstead to counter-attack and establish a winning lead.
The 25-year-old Otley rider eventually crossed the line 25 seconds clear of Pooley in second with South Africa's Ashleigh Pasio in third.
Garner came across the line in 20th place in a time of 2:44.12.