Ngugi's first international success was the victory at the World Cross Country Championships 1986. Over the next three years, he became world champion in cross-country running.
But on the track, he was strong. At the World Athletics Championships 1987 in Rome, he was considered one of the favorites over 5000 meters, after winning his heat. In the final Ngugi took the lead after the second mile, but was überspurtet in the final round and finished in twelfth place disappointing for him.
At the Olympic Games in Seoul in 1988 Ngugi was once again a favorite. After one kilometer, he took over as usual the lead. Although its distance was lower because of the final spurt of his opponents, he still won with thirty meters ahead.
At the 1990 Commonwealth Games in Auckland Ngugi tried exactly the same tactics as in Seoul apply. Though he stumbled in the second round, fell to the ground and 35 meters residue had on the field, he was able to catch up with the leading group and out run a lead of 40 meters. But this time was the boss not big enough, Ngugi was beaten by Australian Andrew Lloyd by eight hundredths of a second.
In 1992, he won for the fifth time at the World Cross Country Championships. Only Kenenisa Bekele has so far often get this item (as of 2008 ).
He has been banned by the IAAF for four years, after an unannounced doping control in his homeland refused in 1993. Ngugi insisted that he had the inspectors can not recognize it as such. In 1995 the ban was lifted because of " exceptional circumstances ". It took into account that Ngugi had not received any instruction from the officials of his country and himself did not have a formal education, with which he would have the rulebook alone can understand. However, the two-year forced break had put an end to his career, in fact.
After a number of business failures he grabbed the end of the 2000s back foot and founded the John Ngugi Foundation, a foundation with which he encourages young athletes.