Next he attended Rugby School where he captained the XV, became their record points scorer and claimed player of the season two years in a row.

Mallinder made his professional debut for Northampton against Bedford Blues on Friday 16 August 2013, as a substitute, in the 56th minute and scored a conversion in the 75th.

[3] As a first team regularly at fly-half or inside centre, Mallinder showed maturity and calm as he took important kicks during the Saints' season, including the winning conversion in Saints' European Rugby Champions Cup play-off final against Stade Francais to secure European Champions Cup rugby for the 2017/18 season.

Mallinder then led England U18 to a 62–5 win over Portugal in the FIRA/AER Championship in April but also scored his maiden try at that level and contributed 30 points in all to his side's three victories in the tournament that brought the European crown.

[6] On 20 April 2017, Mallinder was one of 15 uncapped players named in the senior England tour party to Argentina in June for a 2-test series.