He joined St Mirren on a free transfer in June 2020 and spent two years with the club before returning to the English Football League with Cardiff City in May 2022.

[5] After a man of the match performance in a 3–2 defeat to Southport at the Gateshead International Stadium on his first-team debut, his initial four-week loan spell was extended until the end of the calendar year by manager Ian Bogie.

[10] He made his first League Cup appearance in the quarter-finals at Tottenham Hotspur and spilt a corner while under no pressure, which allowed Nabil Bentaleb to open the scoring in a 4–0 defeat.

[11] In his first start at St James' Park, Alnwick conceded in injury time from Sunderland's Adam Johnson as Newcastle lost the Tyne-Wear derby on 21 December.

[14] In Bradford's next league game, a 2–2 draw with Fleetwood Town at Valley Parade five days later, Alnwick was dropped to the bench as regular goalkeeper Ben Williams returned.

[19] After an unsuccessful trial at Championship side Leeds United,[20] Alnwick joined League One club Port Vale on a five-month contract to cover for the injured Chris Neal on 6 August 2015.

[24] He retained his first-team place upon Neal's recovery from injury, and was named in the English Football League's Team of the Week for "a string of fine saves" in a 2–1 victory over Barnsley on 14 November.

[25] His good form led to him being linked with moves to Wigan Athletic and Preston North End in the January transfer window, though he said that he was "over the moon" to stay with Port Vale.

[35] In December 2019, local newspaper The Sentinel ran a poll of fans to debate Port Vale's best goalkeeper of the 2010s; Alnwick finished in third-place with 26% of the vote, behind Scott Brown (44%) and Chris Neal (30%).

[40] On 22 October 2017, he conceded two goals in a defeat to Motherwell at Hampden Park in the Scottish League Cup semi-finals and was criticised for an inability to command his area.

[44] He joined "the Iron", managed by former Rangers player Stuart McCall, after Gerrard selected McGregor as his first-choice goalkeeper, with Wes Foderingham as his back-up.

[45] He was named on the EFL team of the week after keeping a clean sheet against Shrewsbury Town on 24 September, helping Scunthorpe to their first home win of the season with a series of "key saves".

[52] On 22 June 2020, Alnwick signed a two-year contract with Scottish Premiership club St Mirren; "Buddies" manager Jim Goodwin said he would be "a very able replacement" for Czech goalkeeper Václav Hladký.

[64][65] On 17 May 2022, he was announced as Championship club Cardiff City's second signing of the summer transfer window, where manager Steve Morison was keen to conduct business early and replace outgoing goalkeeper Alex Smithies; Alnwick agreed a two-year contract.

[66] He was sent off on his league debut for the club, during a 2–0 defeat at Preston North End on 11 March; he had only started the game due to a suspension for regular custodian Ryan Allsop.

[72] Alnwick was capped by England at under-17 and under-18 level and was named in Brian Eastick's squad for the 2011 FIFA U-20 World Cup in Colombia as back-up to Jack Butland and Lee Nicholls.