Mark Stephen Ridley, 53, was handed a 10-month suspended sentence on Tuesday and ordered to complete 70 hours' community service for the crime, which occurred in the middle of the day in May last year.

Ridley went to the back of the Reject Shop in Kippax at 1.30pm, exposed himself, and began masturbating.

A grandmother stumbled across him. She saw the indecent act, but turned her granddaughter away in time.

The woman complained to staff and Ridley left the store, still exposed.

He was later arrested and charged with an act of indecency in the presence of another.

It was not Ridley's first crime of its kind.

In 2012, Ridley was caught masturbating in a car in a public loading zone with his windows down.

He was still on a good behaviour order for the earlier offence when he entered the Reject Shop last year.

ACT Supreme Court Justice Richard Refshauge took into account Ridley's struggles with alcohol and drugs from an early age when sentencing him on Tuesday.

He said Ridley had shown some progress in battling his alcohol addiction, but had recently relapsed into alcohol consumption.

The court found Ridley was remorseful and showed some empathy toward the victim.

Ridley said he wouldn't have done what he did if he'd known a child was there, saying he had a granddaughter of his own.

Justice Refshauge sentenced Ridley to 10 months' imprisonment, which was fully suspended.

Ridley was placed on a good behaviour order for two years, and ordered to do 70 hours community service.

He will also be assessed for an adult sex offender program.

"I think you're starting to recognise that your life needs to get onto another track, especially in relation to alcohol," Justice Refshauge told Ridley.

"I hope that you'll take this opportunity to address the problems that you've had and avoid any further offending."