Ashley Maynard-Brewer arrived in SE7 as a 14-year-old, joining from Western Australian outfit ECU Joondalup after being recommended to the club's then-chief scout Phil Chapple by former Addick Andy Petterson.

Since then, the Australian has impressed at Sparrows Lane, navigating his way through the club's youth ranks and regularly training alongside the first-team, culminating with the goalkeeper making his senior bow in November 2018 against Swansea City U21s in the EFL Trophy.

Maynard-Brewer began his loan journey during the 2018/19 campaign, enjoying short spells with National League South sides Chelmsford City and Hampton & Richmond Borough before joining another National League South outfit in Dulwich Hamlet in August 2019. 

The stopper then signed a loan deal with Dover Athletic in the National League, where he made four appearances prior to the COVID-enforced hiatus.

Maynard-Brewer penned a new three-year deal with the Addicks in October 2020 after his penalty shootout heroics against Brighton & Hove Albion U21s in the EFL Trophy. The keeper was named in Australia's 18-man touring party for Tokyo 2020.

He spent the 2021/22 campaign on loan with Ross County in the Scottish top flight, featuring 17 times.

In June 2022, Maynard-Brewer joined League Two side Gillingham on a season-long loan. He suffered a dislocated shoulder during the Gills’ pre-season friendly against Dover and was subsequently recalled in November 2022.

Maynard-Brewer put pen-to-paper on a new long-term contract with the club in March 2023, committing his future to the club until at least 2026.

In September 2023, the goalkeeper received his first senior call-up for the Australia national team.