Born in Gravesend on 2 October 1991, Sam is a good shot-stopper who collects crosses and distributes well and was between the posts throughout the victorious 2010 FA Youth Cup-winning campaign, conceding just three goals in eight games.
He joined from Millwall when still a schoolboy, featuring increasingly in the youth team despite plenty of competition and made a total of 23 appearances in 2009/10. He also made his debut for the reserves that campaign and in 2010/11 played six times for the second string.
In March 2011, he joined League Two Barnet on loan until May 7. It was a fantastic time for the goalkeeper, as he helped The Bees preserve their Football League status with a last-day 1-0 win against Port Vale, his first clean sheet in seven appearances. Beaten just once, he won four of those seven games, but was also sent off for two yellow cards midway through his stay.
Sam spent the first half of the 2011/12 season on loan at League Two Northampton Town, where he made 25 appearances in all competitions before being recalled to Stamford Bridge. In January 2012 he was loaned to Yeovil Town for the rest of the season, where he made 20 appearances under the watchful eye of Gary Johnson, a manager he played under at Northampton.
He also signed a new Chelsea contract that will keep him at the club until 2014.
Prior to the start of the 2012/13 season, Walker signed for League Two side Bristol Rovers on loan until 2 January 2013.
Shortly after returning to Stamford Bridge, it was announced that Walker would be joining Colchester United on loan for the remainder of the campaign, where he kept nine clean sheets and helped the club preserve their League One status.
Ahead of the 2013/14 campaign, it was announced that Walker would return to Colchester on loan until 19 January.