As a permanent part of the Great Britain Women's 4x100 Relay team 1978-84, she won a total of eight medals in Olympic, World, European, and Commonwealth Championships. She always ran the second 'leg', with her rangy gait, speed, and endurance being ideal for this position. She also occasionally competed in the 4 x 400 metres relay.
Her major results began with a silver in the 4 x 100 relay at the European Championships, and a gold medal at the 1978 Commonwealth Games, again in the sprint relay. In the 1980 Moscow Olympics, she won a bronze medal in the sprint relay, in a British record time. She also made both sprint finals.
In 1982, in the 200m final in the European Championships in Athens, Kathy finished second behind Bärbel Wöckel (East Germany), with a new UK and Commonwealth record of 22.13sec. Two days later, she won a second silver medal, as a member of Great Britain's 4 x 100m relay team, which also finished second behind East Germany. Less than one week later, in London, Smallwood won a 400m in 50.46sec, to set a new UK and Commonwealth record. One month later, at the Commonwealth Games in Brisbane, she finished a close second to Merlene Ottey (Jamaica) in a wind-assisted time of 22.21sec in the 200 metres. At these games, she also won gold in the 4 x 100 metres relay.
At the inaugural World Championships, in 1983, she added more major championship medals to her growing haul: over 200 metres in 22.37 seconds; and a silver in the sprint relay in a time of 42.71 seconds, behind the powerful East German squad. The British team had actually been leading into the last exchange, thanks to a superb 3rd leg by team mate and relay specialist Beverley Callendar.
In 1984, probably her finest season, at the Los Angeles Olympics, she won a further two bronze medals. Firstly, over 400 metres, she smashed the British and Commonwealth record. In the far from ideal Lane 1, she collected another bronze, as part of 4x100 metre relay team featuring her teammates Simone Jacobs (1st leg), Beverley Callendar (3rd leg), and Heather Oakes (anchor leg), clocking 43.11 - well down on the British record she helped set alongside Callendar and Oakes in Moscow 1980. She missed another bronze medal in the 200 metres by a mere 1/100th of a second.
In the 1986 Commonwealth Games, she won silver at 200 metres, bronze at 400 metres, gold in the 4 x 100 metres relay, and silver in the 4 x 400 metres relay.
No British woman has since come close to emulating these achievements. They are the more significant, because many of her rivals after the fall of the "Iron Curtain" were found to have been performing illegally. She retired from competition in 1986 after the European Championships in Stuttgart to start a family with husband Garry Cook. She is currently a P.E. teacher at Mayfield Preparatory School, in Walsall, England.