However, he was to miss the next full offseason build-up following a bout of glandular fever and despite lowering his personal best to 46.8s at the end of the 1974–75 season, Mitchell placed second again to Steven Gee from New South Wales.

Two years later Mitchell competed in his first 1978 Commonwealth Games in Edmonton, Alberta, Canada where, in extremely windy conditions, and having to run four rounds of 400-metre races in 30 hours he proved too strong for his opponents, taking the individual 400-metre gold medal and anchoring the 4x400 metre relay team to third place.

In 1979, Mitchell elected not to contest the World Cup, preferring to remain in Australia to build up for the 1980 Summer Olympics in Moscow in similar fashion to the preparation he followed in 1975.

The race itself was won by Russian Viktor Markin in a European record of 44.60s (the fastest time in the world for three seasons), while Mitchell finished strongly to take the silver medal in 44.84s.

He then continued to race in Europe, where he frequently met American athletes who had not contested the Olympic Games following America's boycott of the meeting over the Soviet invasion of Afghanistan in 1979.

The following year, Mitchell accepted a position in England as the Area Manager - UK & Scandinavia for the Ansell Glove Company and athletics took a back seat.

Mitchell then elected to remain in Australia to prepare for one final attempt at the Olympics; however, his domestic season was disrupted by an achilles tendon injury which resulted in 7 weeks without running.

Prior to leaving for the Games, Mitchell's achilles injury had mended and his time trials were faster than those produced before the 1980 Olympics where he won the silver medal.