Domestic Championships Record
2005 – 1st AAA U20 Indoor Championships 60m, 2nd 200m, 1st AAA U20 Championships 100m
2006 – 2nd AAA Indoor Championships 60m
2007 – 3rd Norwich Union European Indoor Champs Trials 60m, 1st BUSA Championships 60m, 1st England Athletics U23 Championships 100m
2008 – 2nd Norwich Union World Indoor Champs Trials 60m, 2nd Olympic Trials 100m
2009 – 3rd Aviva UK Indoor Championships 60m
2009 – 1st Aviva World Trials and National Championships 100m

Major Championships Record
Olympic Games – 2008 4qf 100m (dq 4x100m)
World Championships – 2009 100m 5th qf

Other noteworthy performances
2005 – 2nd European Junior Championships 100m (DNF 4x100m)
2007 – 1st European U23 Championships 100m, 1st World University Games 100m (4th 4x100m)
2008 – 7th World Indoor Championships 60m
2009 – 4th European Indoor Championships 60m

Simeon made a big breakthrough in 2005 when he won a junior sprint double indoors and outdoors over 60m and 100m at the AAA Under 20 T&F Championships. He was also the fourth fastest junior in the world that year and took the silver medal in the 100m in the European Junior T&F Championships.

In 2007 he beat the favourite to take the 100m gold at the European Under 23 T&F Championships in a personal best time of 10.10 secs. One month later he went on to take gold in the World University Games in Bangkok.

In 2008 at the UK Championships (Olympic Trials) Simeon picked up a silver medal in the 100m in a personal best time of 10.03 secs which ranked him fourth on the UK all-time best list. He was selected for the British Olympic team but picked up an ankle injury which prevented him from performing at his best in Beijing. Earlier that year he qualified for the final of the World Indoor Championships.

Williamson underlined his indoor ability at the 2009 Birmingham Indoor Grand Prix when setting two lifetime bests: 6.56 in the heats followed by victory in the final with a personal best of 6.53 to move to eighth on the UK all-time list, but was disappointed with his fourth place finish at the European Indoor Championships, when he had expected "nothing less than silver".

At the 2009 UK Championships Simeon vowed to win the 100m and he did in emphatic style, running 10.05 secs into a head wind of -1.8 and winning by a huge margin to take on the mantle of British number one. He went on to lead the British 4x100m relay team to bronze medals in the World T&F Championships in Berlin.

In 2010 he picked a knee injury while training in Jamaica and had to have an operation which ruled him out for the entire season.

He is also second cousin to high jumper Olympic silver medalist Germaine Mason