Smithies was born in Huddersfield, West Yorkshire. He grew up in Golcar, in the Colne Valley area of Huddersfield. He attended Colne Valley High School, Linthwaite. He grew up supporting Huddersfield Town. Smithies previously played football for Westend Juniors in Huddersfield's local leagues, and he joined the Huddersfield Town academy at the age of 8.
Having come through the Academy of the club he grew up supporting, Smithies made his first team-debut on 5 December 2007. He came on as a 76th minute substitute for the red-carded Matt Glennon during Huddersfield Town's 4–1 defeat by Southend United at Roots Hall. His first full start came in the 4–0 defeat at Leeds United on 8 December, after Glennon lost an appeal against his dismissal and Town were refused permission to loan an emergency goalkeeper on the grounds that Smithies was registered as a professional.
Smithies' first appearance at the Galpharm Stadium for a first-team match was on 6 December 2008. In this game, caretaker manager Gerry Murphy dropped regular goalkeeper Matt Glennon in favour of the young custodian, who went on to help Huddersfield to a 2–1 victory over Walsall. The first clean sheet he kept for the club came on 13 December 2008. In the 1–0 win over Southend United, Smithies saved a penalty from James Walker, 10 minutes from time. Smithies became the first choice keeper for the remainder of the 2008–09 season and, after the arrival of manager Lee Clark, made the number one jersey his own.
Throughout the 2009–10 season, Smithies was the only player to play in each and every league and cup game for Huddersfield. He made 48 starts, 17 of which resulted in a clean sheet for the Terriers. His contribution in helping Huddersfield to a final position inside the League One playoffs was reflected by him winning the club's Young Player of the Year Trophy.
In conceding just 56 goals in such a competitive campaign, he won both praise and admiration from many quarters; earning the Football League's 'Young Player of the Month' award in February 2010 along the way. On 17 November 2009 Smithies was linked with Stoke City after a scout from the club was the latest in a long string of Premier League interest to be seen at the Galpharm. This interest was played down by Lee Clark and Tony Coton, and Smithies insisted he was happy to be at Huddersfield.
Smithies' 2010–11 campaign was punctuated by injury, restricting him to just 27 appearances in all competitions throughout the season. His performances, on the occasions he did play, earned him a call up to the England under-21 squad, although he was unable to play - again due to injury.
In January 2011, he was linked with a £1.5m move to Aston Villa, after Villa manager Gérard Houllier was said to have been impressed with his performance against Sheffield Wednesday.
Further speculation relating to his future at the club followed Huddersfield's 3–0 defeat against Peterborough United in the League One play-off final, and he was linked with a move to Leicester City, along with teammate Lee Peltier. However, when Leicester had a bid for Kasper Schmeichel accepted by Leeds United in June, speculation that Smithies could be heading for Leeds was rejected by Huddersfield chief executive Nigel Clibbens.
Smithies' absence from the squad during 2010–11 through injury allowed Ian Bennett to become first choice goalkeeper, backed up by fellow veteran Nick Colgan. His injury meant he remained sidelined for the first half of the season, and had to wait until 14 February 2012 to make his first appearance in the 2011–12 campaign. This came in the 1–0 home defeat to Sheffield United, a result which would cost manager Lee Clark his job the following day. This was followed by his first clean sheet of the campaign in Huddersfield's 2-0 home win over Exeter City on 25 February 2012. During the second leg League One play-off game against Milton Keynes Dons he came on as a substitute for Bennett, who had gone off with an injured hand. Town lost the game 2-1 but reached the final against Sheffield United 3-2 on aggregate. Smithies started in the final, which finished 0-0 thanks to some excellent saves from the young custodian. In the Penalty shootout Smithies performed more heroics, saving three of the Sheffield penalties. After 10 penalties each, the sides were still inseparable, and Smithies and opposite number Steve Simonsen stepped up to take the next penalties for their sides. Smithies scored in emphatic fashion, before Simonsen put his penalty over the bar to give promotion to the terriers.
On 17 August 2012, Smithies began his debut season in the Championship with Huddersfield in a 1-0 away defeat at eventual league winners Cardiff City. He went on to start all 46 of Huddersfield's Championship fixtures, as well as all but one of the clubs five cup matches during the season, conceding 79 goals in the process.
Smithies started his second season in the Championship how he started his first, by starting all Huddersfield's opening eleven fixtures in all competitions, conceding 10 goals during that time. On 25 September 2013, Smithies agreed a new one-year contract extension at the club, keeping him at the John Smith's Stadium at least until the summer of 2016, with the Club having the option for a further year to June 2017.
Smithies started his third season in the Championship as first choice goalkeeper, ahead of Joe Murphy. He conceded four goals on the opening day in a 4-0 home defeat to Bournemouth, but did save a penalty. The game would prove to be manager Mark Robins' last for the club.
On 20 August 2015, Smithies joined Queens Park Rangers for an undisclosed fee, signing a three-year deal.
He has four caps for the England U-16's and 10 caps for the England U-17's including five during the 2007 FIFA U-17 World Cup in South Korea. In this competition he helped England U-17's make it to the Quarter-finals, before they were eventually eliminated by Germany after a 4–1 defeat.
On 20 November 2007, Smithies made his debut for the England U-18 team, where he played 45 minutes in their 2–0 win against Ghana U-18's at Gillingham's Priestfield Stadium. In November 2008, Smithies received his first England U-19 call up for a friendly against Germany U-19. His first game for the U19's came against Spain U-19 at Bournemouth's Dean Court ground on 10 February 2009, in a 3–0 defeat.
On 24 August 2010, Smithies received his first call-up to the England U-21's for their matches against Portugal U-21's and Lithuania U-21's.As of match played by 8 May 2017
On 2 April 2009, Smithies was one of a select few young British footballing talents assessed by "Sky Sports Scout", a service offered by Sky Sports for finding the best young talent in the country. He was given a rating of 65/80, a current valuation of £4million and labelled as "on his way to the top."
Smithies was named as the Football League's Young Player of the Month for February 2010 after keeping three clean sheets in seven undefeated Huddersfield Town games.