Sam Walker joined Chelsea from Millwall's youth team in 2007 for a fee of around £175,000. He is described as a "good shot stopper who collects crosses and distibutes well".

Sam joined Norwich on loan in March 2009 playing for the Under-19s in their 1-0 defeat at Reading on 21st March 2009 after Jed Steer was called up by England and Declan Rudd and Sam Grieverson both sidelined through injury

Walker was part of the Chelsea youth team that won the 2010 FA Youth Cup defeating Aston Villa in the final

On 24th March 2011, Sam joined Barnet on loan until the end of the season, becoming new manager Martin Allen's first signing. He made 7 appearances for Barnet

In July 2011, he joined Northampton on a season long loan but returned to Chelsea on 23rd December 2011, after making 21 league appearances, when the new Cobblers boss Aidy Boothroyd signed Shane Higgs as first choice keeper.

Walker joined Yeovil on loan on 19th January 2012. He made 20 appearances for Yeovil.

On 14th August 2012, Walker was loaned to Bristol Rovers until 2nd January 2013. He made 11 league appearances for Rovers.

On 14th January 2012, Walker joined Colchester on loan until 27 April 2013. 21-year-old was influential in the U's League One survival bid last season, keeping seven clean sheets from his 17-game loan spell. He rejoined Colchester on loan on 19th July 2013, until 19th January 2013. He played 26 times for the U's during the loan spell, keeping seven clean sheets

On 20th January 2013, Walker signed for Colchester on an 18 month contract. "He is another young and hungry footballer who has decided to come here and play, which is great testament to what we are doing here at Colchester." said boss Joe Dunne

After his contract with Colchester expired in the summer of 2018, he signed a three year deal with Reading on 25th July 2018., reuniting with Reading manager Paul Clement whom he worked with at Chelsea's youth team. He had made 213 appearances for Colchester.

On 23rd December 2020 Walker joined Blackpool on an emergency one week loan as cover for Blackpool goalkeeper Chris Maxwell who tested positive for Covid-19. He made two appearances for the Tangerines before returning to Reading who promptly loaned him to AFC Wimbledon on 11th January 2021 for the remainder of the 2020/21 season.

He was released by Reading on 11th May 2021. He had played 14 games for Reading.

In July 2021, he signed for Scottish Championship side Kilmarnock. He left them in the summer of 2023, having played 28 times in the 2022/23 season.

On 16th October 2023 he signed for Charlton Athletic on a short term deal until January 2024. He will provide cover while Harry Isted recovers from a knee injury.

After his contract with Charlton ended, he signed an 18 month contract with Bradford City on 11 January 2024 and will be their number one after Harry Lewis moved to Carlisle.