He began to work in the NXT brand as part of the stable Sanity, along with Nikki Cross, Eric Young and Alexander Wolfe.

[14] He stated that as a child he was attracted to professional wrestling because of the "larger than life" personas and face paint of wrestlers such as Sting and The Ultimate Warrior.

[15] He cites fellow countryman Fit Finlay as a huge influence on his wrestling style and inspiration for his success.

[15] After training at NWA Scotland, Mackle debuted for the Scottish Wrestling Alliance at the age of 20 under the ring name Damian O'Connor.

He originally wrestled in Scotland and the North of England as a tag team called "Britain's Most Wanted" with Scott Renwick.

[30] In 2016, he wrestled international wrestlers "Speedball" Mike Bailey,[31] Roderick Strong,[32] Dalton Castle,[33] Big Daddy Walter[34] and Matt Sydal.

[35] In October 2015, Damo debuted for Global Force Wrestling (GFW) on their UK Invasion tour, losing to Bram.

[39] On the 1 March episode of IMPACT Wrestling, Big Damo unsuccessfully challenged Eric Young for the TNA King of the Mountain Championship.

[40] At TNA One Night Only: Joker's Wild, Big Damo and Jimmy Havoc were defeated by Drew McIntyre and Mike Bennett.

[46] Mackle made his in-ring debut at an NXT live event on 11 November, defeating Rich Swann under the ring name Damian O'Connor.

[49] On the 18 January episode of NXT, Dain joined SAnitY in an official capacity after attacking Tye Dillinger and accepting Sawyer Fulton's jacket from Eric Young.

[50] On the 25 January episode of NXT, Dain appeared as part of SAnitY for the first time, attacking Chris Atkins by Young's order.

[51] On the 8 February episode of NXT, Dain teamed with Young and Alexander Wolfe to defeat Dillinger, No Way Jose and Roderick Strong.

[53] At WrestleMania 33, Dain made it to the final three in the André the Giant Memorial Battle Royal before being eliminated by eventual winner Mojo Rawley.

[65] After a near four-month hiatus from television, SAnitY returned on the 2 April 2019 episode of SmackDown LIVE, losing a three-on-one Falls Count Anywhere match to The Miz.

On June 25, 2021, Dain was released from his WWE contract, this as part of a fourth round of layoffs executed by the brand due to the effect of the COVID-19 pandemic.

[8][74] On the June 4, 2022 episode of NJPW Strong, Big Damo made his debut in New Japan Pro Wrestling losing to Tomohiro Ishii.

[75] On January 17, 2019, Mackle married fellow WWE wrestler, Nicola Glencross, known by her ring name Nikki Cross.

Damo has also appeared on the YouTube wrestling quiz show Quizzlemania, hosted by Adam Blampied on the "Parts Funknown" channel.