[10] Leach made his Somerset debut in a two-day match against the touring South Africans in July 2012, claiming the prized wicket of Hashim Amla.

[22] After losing his place to Adil Rashid and then Moeen Ali in the 2018 home summer, Leach was recalled for the winter tour of Sri Lanka.

In an unusual circumstance, with one over left to play in the day during the second test, Leach opened England's innings as a nightwatchman, surviving till stumps.

Leach scored one run, tying the game, in a partnership of 76 with Ben Stokes, to help England win the third Ashes test at Headingley, by one wicket.

He regularly cleans them through his innings, and has received free Specsavers glasses for life following his heroics supporting Ben Stokes in the dramatic Headingley test.

[28] Leach played the rest of the series including a pivotal role in the final test, bowling England to victory with figures of 4–49.

[32][33] On 17 June 2020, Leach was included in England's 30-man squad to start training behind closed doors for the Test series against the West Indies.

Therein he took 28 wickets across the six tests,[38][39] but did not retain his place for the summer; England instead opting not to use a spinner against New Zealand and then preferring Moeen Ali for the Pataudi Trophy.

Following the sudden retirement of Ali, Leach was included in the 2021–22 Ashes series squad and was selected ahead of Stuart Broad for the first test at The Gabba despite the pitch being a "green top" which favoured the seamers.

He was less prolific against South Africa, as the low-scoring nature of the series meant he was often not required to bowl, but in the 2022–23 winter he took 25 wickets across five tests against Pakistan and New Zealand, including a five-wicket haul in Wellington.

[51] In October 2024, Leach returned to the England team for the first match of their three-Test series against Pakistan, taking seven wickets in the contest including 4/30 in the hosts' second innings.