He made his first team debut on 2 October 2002 in a League Cup second round tie away at Leyton Orient, starting in the 3–2 win at Brisbane Road.

[10] On 7 February 2006, having not played for Birmingham since 28 December 2002 nor any team since 10 January 2004, Sadler made his return in a 2–1 home win over Reading in an FA Cup fourth round replay, coming on for Mathew Birley after 59 minutes.

[15] Sadler played a role in a match against Coventry City on 31 October 2006 when he provided assist for Nicklas Bendtner to score the only goal of the game with a win.

[21] After missing out the first two matches, Sadler made his first appearance of 2007–08 when he came on as a substitute for Fabrice Muamba in the 80th minute, in a 1–0 loss against West Ham United on 18 August 2007.

[22] Sadler then provided assist for Garry O'Connor in the 27th minute, in the first round of the League Cup, in a 2–1 win over Hereford United on 28 August 2007.

[33] After missing out five matches, Sadler regained his first team place and then provided assist for Tamás Priskin to score the first goal of the game, in a 3–0 win over Southampton on 18 October 2008.

[36] Sadler eventually fell from grace at Vicarage Road and made his final appearance for Watford on 26 December 2008 in a 2–4 home defeat to Bristol City.

According to manager Graham Turner, the player seemed reluctant initially to play in the fourth tier of English football but was won over by the club's ambition.

[48] Since then, Sadler made an impressive display at Shrewsbury Town, having established himself in the first team and expressed interests of extending his loan spell at the club for the remainder of the season.

[57] Against Hartlepool United seven days later, Walsall took an early lead via Jon Macken's penalty after Sadler was fouled; the match ended as a 1–1 draw.

"[59] Against Huddersfield Town on 5 November, Sadler crossed the ball for Jamie Paterson's equaliser that ended a three-match losing streak.

[60][61] He was sent off for a second bookable offence against Charlton Athletic on 10 December,[62] so was suspended for the FA Cup second round replay against Dagenham & Redbridge which Walsall lost in a penalty shootout;[63][64] it would remain the only match he missed all season.

[66] Facing Colchester United on 27 March 2012, in his 193rd league game (222nd in all competitions), Sadler scored his first senior goal, breaking what BBC Sport described as "one of football's longer droughts".

[67][68] The 3–1 win lifted Walsall out of the relegation places,[67] and Florent Cuvelier's equaliser against Huddersfield on 28 April from Sadler's cross confirmed their safety.

[70][71] On 8 June 2012, Sadler joined newly promoted League One side Crawley Town on a free transfer after turning down a new two–year contract with the club.

[72] Sadler made his debut on 14 August, starting in a League Cup match as a left back against Championship side Millwall which ended 2–2, with Crawley winning 4–1 on penalties.

[73] Four days later, on 18 August 2012, Sadler made his league debut for Crawley Town, in the opening game of the season, in a 3–0 win over Scunthorpe United.

[81] Sadler joined newly promoted Championship side Rotherham United on a free transfer from Crawley Town on 17 June 2014.

[83] After appearing five times in the pre–season friendly matches, it wasn't until on 26 August 2014 for Sadler made his Rotherham United debut in the second round of the League Cup, which saw them lost 1–0 against Swansea City.

[91] Sadler then provided assist for Conor Wilkinson on 21 March 2015, to score the only goal in the game in a 1–0 win over Crewe Alexandra, to end the club's three match losing streak.

[96] A regular in the side in the early part of the season, Sadler scored his first goal for the club in a 2–1 defeat to Chesterfield,[97] but lost his place in the first-team after a run of poor personal form.

[98] Having been recalled for an FA Cup first round victory at non-league Gainsborough Trinity, he was praised for his "first class attitude" and determination by manager Micky Mellon.

Immediately after the final match of the 2018–19, Sadler revealed that he would be leaving the club at the end of his contract, having clocked up over 200 appearances in all competitions, over two separate spells.

[112] On 7 January 2025, with Walsall sitting top of the league, ten points clear of second place, Sadler signed a new three-and-a-half year contract.